[libmultipath] cache sysfs_devices

[libmultipath] cache sysfs_devices

this one fixes the described bug of paths coming back as different block
devices:

Keep a list of sysfs devices for sysfs_device_get() so uev_trigger() can look
up the necessary information for proper path removal in case of a 'remove'
uevent - the sysfs files in the filesystem might be long gone at this point.

This also plugs a memory leak where we'd malloc space for the same sysfs device
over and over again for every processed uevent.