multipathd: Implement systemd watchdog integration

multipathd: Implement systemd watchdog integration

In the past there have been several instances where multipathd
would hang with the checkerloop as some path checker might not
be able to return in time.
This patch now activates the watchdog feature from systemd
to shutdown (and possibly restart) multipathd in these
situations.
Due to a bug in systemd watchdog integration only works
correctly with later version (> 206), so watchdog integration
has been disabled per default.

Signed-off-by: Hannes Reinecke <hare@suse.de>