If a proc has P_WEXIT set do not stop it, let it exit since it is already

If a proc has P_WEXIT set do not stop it, let it exit since it is already
mostly dead.

This is more like belts and suspenders since a proc in exit1() will not
receive signals anymore and so proc_stop() should not be reachable. This
is even the case when sigexit() is called and a coredump() is happening.
OK mpi@