There's a race between runtime.goexitsall killing all OS processes
of a go program in order to exit, and runtime.newosproc forking a
new one.  If the new process has been created but not yet stored
its pid in m.procid, it will not be killed by goexitsall and
deadlock results.

This CL prevents the race by making the newly forked process
check whether the program is exiting.  It also prevents a
potential "shoot-out" if multiple goroutines call Exit at
the same time, which could possibly lead to two processes
killing each other and leaving the rest deadlocked.

Change-Id: I3170b4a62d2461f6b029b3d6aad70373714ed53e
Reviewed-on: https://go-review.googlesource.com/21135
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>