Adopt the OpenBSD syntax for setting and filtering on VLAN PCP values. This
introduces two new keywords: 'set prio' to set the PCP value, and 'prio' to
filter on it.
Reviewed by: allanjude, araujo
Approved by: re (gjb)
Obtained from: OpenBSD (mostly)
Differential Revision: https://reviews.freebsd.org/D6786