lib/tevent: handle tevent_common_add_signal on different event contexts.
I don't know if this is a problem in real life.
The code assumes there's only one tevent_context; all signals will notify
the first event context. That's counter-intuitive if you ever use more
than one, and there's nothing else in this code which prevents it AFAICT.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(cherry picked from commit
be4ac227842530d484659f2db683453366326d8b)
(cherry picked from commit
792ab5c34a20bd2b292b642dc96cae62e5ad1ce0)