mount: add support for mixed usage of SPECes

mount: add support for mixed usage of SPECes

This patch improves a way how the mount works with SPECes (devname,
LABEL or UUID) and nodes (mountpoints) from command line.

The patch adds support for mixed usage of SPECes -- it means on
command line is different SPEC than in the /etc/fstab file. For
example:

command line:
   mount LABEL=foo
fstab:
   UID=915b048a-998d-4ee5-9e6b-7fcaaf34d3c5 /mnt/foo auto default

The mount command doesn't strictly require same SPEC on command line
and in fstab anymore. You can be more creative and mix UUID, LABEL
or device name.

This implementation is more effective that convert *all* entries from
/etc/fstab to a real devnames.

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