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

(cherry picked from commit 1e7fe2fbb9c0bed230d8db94d7444a90ca5ce199)