pfctl: Add missing 'va' code point name

Add the 'va' (voice-admit, RFC5865) symbolic name.

Reviewed by: rgrimes, gbe (man page)
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29069

(cherry picked from commit b4e3f3c2de6e6dc614f99615e50d0d87f3367ca0)