We previously used execlp to execute seatd, which had the effect of
searching PATH for the executable. This allowed the caller to control
what executable was run, which is bad if SUID has been set.
Instead, expose the absolute install path for seatd from meason as a
define, and use that in a call to execv.
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.
