When a process group leader exits, all of the processes in the group are
sent SIGHUP and SIGCONT if any of the processes are stopped. Currently this
behavior is triggered for any type of process stop including ptrace() stops
and transient stops for single threading during exit() and execve().
Thus, if a debugger is attached to a process in a group when the leader
exits, the entire group can be HUPed. Instead, only send the signals if a
process in the group is stopped due to SIGSTOP.

PR: 201149
Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D3681