Prevent error messages to stderr caused by libblkid calling libdevmapper

Prevent error messages to stderr caused by libblkid calling libdevmapper

Make the libdevmapper fail quietly if blkid is called without root
privileges or the kernel does not include device mapper support.
(What is the device mapper _library_ doing writing to stderr, anyway?)

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>