multipathd: skip uninitialized devices during reconfiguration

libudev has a separate filter setting to skip uninitialized
devices during enumeration.

References: bnc#908915

Signed-off-by: Hannes Reinecke <hare@suse.de>
multipathd: memory overflow printing help text

When printing out the help text the allocated buffer was never
checked for the overall length, causing a silent memory overflow
when using 'multipathd -k'.

References: bnc#909742

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