Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add pwait utility, which waits for any process to terminate.

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@