The exit status will be 124, as the timeout(1) utility uses.
Reviewed by: jilles
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D9697
This is similar to the Solaris utility of the same name.
Some use cases:
* rc.subr's wait_for_pids
* interactive use, e.g. to shut down the computer when some task is done
even if the task is already running
Discussed on: hackers@