SEATD_LOGLEVEL was used to set the loglevel despite already having
getopt in place. Remove the environment variable and make a command-line
argument for it instead.
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).
This adds the ability to specify the number of an fd that is inherited
by the process as open. Once seatd is read to serve requests, it will
write a single newline and close the fd.
