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When ueventd creates a device node, it may also create one or more
symlinks to the device node.  These symlinks may be the only stable
name for the device, e.g. if the partition is dynamically assigned.
Extend the label file backend to support looking up the "best match"
for a device node based on its real path (key) and any links to it
(aliases).  The order of precedence for best match is:
1) An exact match for the real path (key), or
2) An exact match for any of the links (aliases), or
3) The longest fixed prefix match.

Change-Id: Id6c2597eee2b6723a5089dcf7c450f8d0a4128f4
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>