ipfw: Add missing 'va' code point name

Per RFC 5865, add the 'va' (VOICE-ADMIT, 101100) symbolic name.

Reviewed By: melifaro, pauamma
Differential Revision: https://reviews.freebsd.org/D37508
MFC after: 2 weeks

(cherry picked from commit bdd60b224fa461a5849f60575afdb458613f4ccd)