src/libadacl.c: rename variables in acl_update_entry() for clarity.

The updated copy of the entry in the acl_update_entry() function was
called simply "entry", which makes it hard to mentally separate from,
say, the "existing_entry" that is used in the same place. The same
problem exists with names like "entry_tag" with respect to
"existing_tag", and so on.

This commit renames "entry" to "updated_entry" and all of the other
variables like "entry_tag" to "updated_tag". That should make it a
little clearer which entry is the new one.