libblkid: add blkid_evaluate_spec()

libblkid: add blkid_evaluate_spec()

The function blkid_evaluate_tag() is useful for tags only (e.g.
LABEL=foo). But we also need to address devices by tags OR paths in
many utils.

The function blkid_evaluate_spec() support this functionality without
extra care about the way how device is addressed. The tags as well as
paths are converted to the standardized device path.

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