Fix #918: add /dev/mapper symlinks for dm-* devices

Fix #918: add /dev/mapper symlinks for dm-* devices

Mount canonicalizes paths unless the -c option is used. This
is mostly fine but for device-mapper nodes (/dev/dm-*) it'll
fetch the /dev/mapper/* path and pass that to the mount
system call resulting in /proc/mounts showing the
/dev/mapper path. This is neither the one we provided (since
we use /dev/$vg/$lv), nor the one userspace tools will find
in /dev currently.
Since the dm-* paths are rather inconvenient to look at we
decided to keep mount's behavior and compensate by providing
the /dev/mapper symlinks for devices via the autodev hook.