gmain: Document constraints on waitpid

gmain: Document constraints on waitpid

Applications that use glib should not invoke waitpid with a first
argument that is nonpositive, because when such a waitpid is run in
one thread and glib waits for a subprocess in another, there is a race
condition, and the former waitpid can reap a process that was intended
for the latter.  Mention this in the documentation for
g_child_watch_source_new, and in the diagnostic generated by
g_spawn_sync when its waitpid fails with errno equal to ECHILD.

Signed-off-by: Colin Walters <walters@verbum.org>
http://bugzilla.gnome.org/show_bug.cgi?id=687075