All silver subcommands look up the parent process’s name, and use
that, verbatim, to identify the shell in use.  This has several
problems:

 - On Windows, the parent process’s name may or may not end with
   “.exe”; there was special case logic to handle “powershell.exe”
   versus “powershell” but not “bash.exe” versus “bash”.  Relatedly,
   the parent process’s name ought to be interpreted case-
   insensitively on Windows; BASH.EXE is the same as bash.exe.

 - On Unix, there is a notion of a _login_ shell (invoked directly by
   login(1) for console logins, or sshd(1) for remote logins); these
   are identified by a leading dash in their process name, *not* by a
   special option.  There was no code to handle this at all (bug reujab#67).

 - sysinfo is slow, because it reads a whole bunch of files in
   /proc whether it’s going to need the data within or not.
   Some but not all of this overhead can be eliminated by using
   `System::new_with_specifics(RefreshKind::new())` instead of
   `System::new_all()`, but even then the wall-clock time for
   `silver init` drops from 0.021s to 0.002s (release build)
   if sysinfo is not invoked at all.

This patch addresses all these problems, as follows:

 - There is a new command line option `--shell` which may be used to
   explicitly set the shell in use.  Also, support for the
   `SILVER_SHELL` environment variable (used for this purpose in 1.1
   and previous) is restored.  sysinfo is only consulted if neither
   the command line option, nor the environment variable, is
   available.

 - No matter where we get the process name from, it is lowercased,
   a trailing “.exe” is removed, a leading dash is removed, and any
   leading directory names are also removed (bug reujab#16).

 - The initialization scripts set `SILVER_SHELL`, so sysinfo will be
   consulted at most once per session, not once per command.

I also reorganized the code in the Command::Init branch of main for
readability’s sake, and made the “unknown shell” error message not be
a panic.  (Panics are for _bugs_, not for errors with a cause outside
the program.)

All silver subcommands look up the parent process’s name, and use
that, verbatim, to identify the shell in use.  This has several
problems:

 - On Windows, the parent process’s name may or may not end with
   “.exe”; there was special case logic to handle “powershell.exe”
   versus “powershell” but not “bash.exe” versus “bash”.  Relatedly,
   the parent process’s name ought to be interpreted case-
   insensitively on Windows; BASH.EXE is the same as bash.exe.

 - On Unix, there is a notion of a _login_ shell (invoked directly by
   login(1) for console logins, or sshd(1) for remote logins); these
   are identified by a leading dash in their process name, *not* by a
   special option.  There was no code to handle this at all (bug reujab#67).

 - sysinfo is slow, because it reads a whole bunch of files in
   /proc whether it’s going to need the data within or not.
   Some but not all of this overhead can be eliminated by using
   `System::new_with_specifics(RefreshKind::new())` instead of
   `System::new_all()`, but even then the wall-clock time for
   `silver init` drops from 0.021s to 0.002s (release build)
   if sysinfo is not invoked at all.

This patch addresses all these problems, as follows:

 - There is a new command line option `--shell` which may be used to
   explicitly set the shell in use.  Also, support for the
   `SILVER_SHELL` environment variable (used for this purpose in 1.1
   and previous) is restored.  sysinfo is only consulted if neither
   the command line option, nor the environment variable, is
   available.

 - No matter where we get the process name from, it is lowercased,
   a trailing “.exe” is removed, a leading dash is removed, and any
   leading directory names are also removed (bug reujab#16).

 - The initialization scripts set `SILVER_SHELL`, so sysinfo will be
   consulted at most once per session, not once per command.

I also reorganized the code in the Command::Init branch of main for
readability’s sake, and made the “unknown shell” error message not be
a panic.  (Panics are for _bugs_, not for errors with a cause outside
the program.)

All silver subcommands look up the parent process’s name, and use
that, verbatim, to identify the shell in use.  This has several
problems:

 - On Windows, the parent process’s name may or may not end with
   “.exe”; there was special case logic to handle “powershell.exe”
   versus “powershell” but not “bash.exe” versus “bash”.  Relatedly,
   the parent process’s name ought to be interpreted case-
   insensitively on Windows; BASH.EXE is the same as bash.exe.

 - On Unix, there is a notion of a _login_ shell (invoked directly by
   login(1) for console logins, or sshd(1) for remote logins); these
   are identified by a leading dash in their process name, *not* by a
   special option.  There was no code to handle this at all (bug reujab#67).

 - sysinfo is slow, because it reads a whole bunch of files in
   /proc whether it’s going to need the data within or not.
   Some but not all of this overhead can be eliminated by using
   `System::new_with_specifics(RefreshKind::new())` instead of
   `System::new_all()`, but even then the wall-clock time for
   `silver init` drops from 0.021s to 0.002s (release build)
   if sysinfo is not invoked at all.

This patch addresses all these problems, as follows:

 - There is a new command line option `--shell` which may be used to
   explicitly set the shell in use.  Also, support for the
   `SILVER_SHELL` environment variable (used for this purpose in 1.1
   and previous) is restored.  sysinfo is only consulted if neither
   the command line option, nor the environment variable, is
   available.

 - No matter where we get the process name from, it is lowercased,
   a trailing “.exe” is removed, a leading dash is removed, and any
   leading directory names are also removed (bug reujab#16).

 - The initialization scripts set `SILVER_SHELL`, so sysinfo will be
   consulted at most once per session, not once per command.

I also reorganized the code in the Command::Init branch of main for
readability’s sake, and made the “unknown shell” error message not be
a panic.  (Panics are for _bugs_, not for errors with a cause outside
the program.)