libxtables: Extend MAC address printing/parsing support

Adding a parser which supports common names for special MAC/mask
combinations and a print routine detecting those special addresses and
printing the respective name allows to consolidate all the various
duplicated implementations.

The side-effects of this change are manageable:

* arptables now accepts "BGA" as alias for the bridge group address
* "mac" match now prints MAC addresses in lower-case which is consistent
  with the remaining code at least

Signed-off-by: Phil Sutter <phil@nwl.cc>