daemons: write pid file even when told not to daemonize (firefly)

systemd wants to run daemons in foreground, but daemons wouldn't write
out the pid file with -f.  Fixed.

Signed-off-by: Alexandre Oliva <oliva@gnu.org>
(cherry picked from commit bccb0eb64891f65fd475e96b6386494044cae8c1)