For processes marked with PS_STOPEXIT, update the process's p_waited
value, and update its parent's p_nstopchild value when marking the
process's p_stat to SSTOP.  The process needed to be SACTIVE to get
here, so this transition represents an additional process for which
the parent needs to wait.

Fixes PR kern/50308

Pullups will be requested for:

      NetBSD-7, -6, -6-0, -6-1, -5, -5-0, -5-1, and -5-2

(pgoyette)