Mimick shells and exit with a status >128 if our child has been
signalled. Exiting with 128 + signal number is what most shells do
(POSIX only requires them to exit with >128).
The command indexing had not been updated afer the introduction of
getopt, so combining a command with flags would fail.
Add error handling for if no command was specified while we're at it.
Instead of relying on seatd's user/group arguments, which require
turning our UID back into a username, just chmod/chown the socket
ourselves once seatd is ready.
We also reduce the permissions to just user access, instead of user and
group like seatd specifies.
