multipathd: no_map_shutdown option

This patchs implements the 'no_map_shutdown' configuration option.
Idea is that the daemon will exit if no maps are found, as then
there's nothing to be done for the daemon anyway.
The daemon will be restarted via socket activation for any CLI
command.
This this option the multipathd.socket unit can be activated always
and won't add to the overall system load.

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