This removes the custom argument parsing logic in favor of using `argh`.
`argh` makes it simpler to add additional subcommands that may or may
not have their own arguments.

This adds a new subcommand `generate-hostname` that is meant to be used
as a settings generator.  It will attempt to read the current IP from
file and resolve it via DNS reverse lookup.  If the lookup is successful
it is used, otherwise the IP is used with dots replaced by dashes to
avoid any confusion from other software that may attempt to read it, e.g
"1.2.3.4" will end up being hostname "ip-1-2-3-4".

This adds a new subcommand `set-hostname` that sets the hostname for the
system by writing it to `/proc/sys/kernel/hostname`

This commit adds the ability to configure the hostname of the system via
a new setting `settings.network.hostname`.  If the setting isn't
populated, at boot time we use the settings generator `netdog
generate-hostname` to populate it.  The generator attempts to resolve
the current IP via reverse lookup, and if unsuccessful uses the IP to
create a hostname in the format "ip-x-x-x-x".

A new service `set-hostname` runs after `settings-applier` and writes
the hostname to `/proc/sys/kernel/hostname` via `netdog set-hostname
$HOSTNAME`.  The service unit file uses an env file that is a template
using the setting `settings.network.hostname` to populate the $HOSTNAME
variable.

This adds the migrations needed for `settings.network.hostname`, one for
the setting, its service and configuration file, and one for the setting
generator metadata.