event: rework sd-event exit logic

event: rework sd-event exit logic

With this change a failing event source handler will not cause the
entire event loop to fail. Instead, we just disable the specific event
source, log a message at debug level and go on.

This also introduces a new concept of "exit code" which can be stored in
the event loop and is returned by sd_event_loop(). We also rename "quit"
to "exit" everywhere else.

Altogether this should make things more robus and keep errors local
while still providing a way to return event loop errors in a clear way.