Allow the ruleset to use the slightly more modern name, 'dscp' rather
than 'tos'. Given that the kernel actively ignores the ECN bits in the
ToS byte 'dscp' is also somewhat more accurate.
While here add the 'va' (voice-admit, RFC5865) symbolic name.
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")