Process group orphaning fixes

I fixed several issues in our handling of the process group job control
state, biggest of which was uncovered by the reaping facility.  The bugs
demostrate itself and underflow (and probably overflow) of pgrp->pg_jobc,
that was reported as panics with added and then removed asserts.

Feel free to participate in review at https://reviews.freebsd.org/D26116.
See the review summary for more detailed description of the bugs and the
patch itself.

Targeted tests were narrowed down by Peter Holm, who also tested the
proposed patch.