bpf: Add an ioctl to set the VLAN Priority on packets sent by bpf

This allows the use of VLAN PCP in dhclient, which is required for
certain ISPs (such as Orange.fr).

Reviewed by: bcr (man page)
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31263