This will make a number of things easier in the future, as well as (finally!)
avoiding the Id-smashing problem which has plagued developers for so long.
Boy, I'm glad we're not using sup anymore. This update would have been
insane otherwise.
return EX_TEMPFAIL if the file was already locked. This makes it easier
to distinguish between lock collisions and failures within the command
being executed.
Also, don't complain if the unlink() fails in the cleanup handler. It
doesn't matter anyway, and it obscured the exit status returned from
the command that was executed.
