syslogd: Centralize operations into a kevent loop

Use kqueue(2) for socket I/O and signal notification. Previously,
select(2) and traditional signal handlers were being used.

This change centralizes all of the async notification delivery into a
single loop so future Capsicum sandboxing will be easier. It also
simplifies the code by removing boiler-plate cruft that comes with the
older interfaces.

Reviewed by: Slawa Olhovchenkov, markj, emaste
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41363