Extend ueventd to use the label-by-symlink support in libselinux.

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.
A corresponding change with the same Change-Id to external/libselinux
introduces selabel_lookup_best_match() to support looking up the "best match"
for a device node based on its real path (key) and any links to it
(aliases).  This change updates ueventd to use this new interface
to find the best match for the device node when creating it.

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