Only kill process group orphans if a remaining member is stopped due to a signal.

[jhb]

Summary

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.

In particular, gdb creates a new process group each time it runs a new
process. If that process forks a child and gdb follows the child, the
child is killed with SIGHUP when the parent exits.

PR: 201149

Test Plan

• Run a simple test program that forks a child but doesn't wait for the child in the parent under gdb with follow-fork-mode set to child and using a breakpoint or catchpoint to ensure the child stops while the parent exits. The child will be killed with SIGHUP when it resumes without this change.