watchfrr, lib: cleanup & delay detaching

watchfrr, lib: cleanup & delay detaching

This cleans up watchfrr to be more "normal" like the other daemons in
terms of what it does in main(), i.e. using the full frr_*() call set.

Also, this changes the startup behaviour on watchfrr to stay attached on
the daemon's parent process until startup is really complete.  This
should allow removing the "watchfrr.started" hack at some point.

Signed-off-by: David Lamparter <equinox@diac24.net>