loop: make uloop_run() return the cancelling signal

loop: make uloop_run() return the cancelling signal

When a process quits in response to a signal it handles, it should to so
be re-sending the signal to itself. This especially important for SIGINT,
as is explained in [1].

uloop currently hides the reason for quitting uloop_run(). Fix this by
returning the signal that caused the loop to quit (or 0 when uloop_end()
was used), so a program using loop an comply with [1].

[1] https://www.cons.org/cracauer/sigint.html

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>