multipathd: don't flood system with sd_notify calls

multipathd: don't flood system with sd_notify calls

DAEMON_RUNNING is only used by checkerloop to indicate that
path checkers are running. checkerloop switches back and forth
between DAEMON_IDLE and DAEMON_RUNNING in every iteration, i.e.
it performs two status changes per second on an idle system.
The repeated sd_notify() calls cause a lot of traffic on dbus as
systemd forwards these messages on the system bus. This can be
seen with systemd.log_level=debug. Better skip these notifications
that don't provide useful information to the user, who is very
unlikely to catch the daemon in "running" state anyway.

Signed-off-by: Martin Wilck <mwilck@suse.com>