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>