don't rely on HOME environment variable for homedir?

[stevengj]

In JuliaLang/IJulia.jl#309, there was a failure that seems to have been caused by the HOME environment variable not being defined (when Julia is launched by IPython in a virtualenv), although we are still trying to pin down the problem.

It occurred to me that relying on this environment variable for the homedir() function might not be completely robust.

• On Unix, we could instead call getpwuid(getuid())->pw_dir
• On Windows, there is SHGetKnownFolderPath with the FOLDERID_Profile option.

We would probably have to write a helper routine in C to access these reliably (e.g. to get struct passwd from pwd.h).

[stevengj]

The next release of libuv will have uv_os_homedir, which we can use for this purpose.