mdmfs(8): Check for other types of helper-program failure

Exiting with a signal should not be treated the same as successful exit with
zero status.

Return signal exit information to the callers via negative integers, to
enable distinction from normal exit statuses. (All consumers that check for
errors don't care what the exact non-zero exit value is -- in such a case
they print a diagnostic message and either continue or bail.)

Additionally, check for unexpected sources of waitpid() wakeup and bail if
we encounter them.

Reported by: lev@
Reviewed by: kib, lev, markj (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17035