* add systemd sd_notify support
roughly based on sd_notify() from systemd and https://github.com/bb4242/sdnotify
only implements `READY=1` and `STATUS=Gunicorn arbiter booted` of the
protocol in the arbiter. in the future, reloads can be notified, and
possibly also other statuses.
see https://www.freedesktop.org/software/systemd/man/sd_notify.html for
more info
sd_notify() is a noop when not run in a systemd service (i.e
NOTIFY_SOCKET environment variable is not set)
Track the use of systemd socket activation and gunicorn socket inheritance
in the arbiter. Unify the logic of creating gunicorn sockets from each of
these sources to always use the socket name to determine the type rather
than checking the configured addresses. The configured addresses are only
used when there is no inheritance from systemd or a parent arbiter.
Fix#1298
