libblkid: rename UUID_LOG to LOGUUID

 * the goal is to make the uuid visible and usable it as mount
   variable/spec, then LOGUUID= seems better than UUID_LOG=

 * we already have PARTUUID=

 * the UUID_RAW and UUID_SUB are not used by mount(8)

Signed-off-by: Karel Zak <kzak@redhat.com>