systemd: add triggers for reloading/restarting services

What we want is to reload services that need to be reloaded after
upgrade (those that are marked with needs-reload or needs-restart), to
handle systemctl daemon-reload too, and to reload udev, sysusers,
tmpfiles, etc.

This all needs to be done in a particular order, which we would ideally
do with different triggers (and triggers with consistent ordering!), but
apk can't do that so instead we have this inefficient solution which
will probably do a bunch of useless processing. ah well

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>
Part-of: !6334