Modify VLAN actions to support setting both VID and priority.

Modify VLAN actions to support setting both VID and priority.

Stripping VLANs is now done through the OFPAT_STRIP_VLAN action (i.e., you
don't specify a magic value in the generirc VLAN action).  Also, it is now
possible to modify the priority bits associated with the VLAN tag through
the OFPAT_SET_VLAN_PCP action.  The OFPAT_SET_DL_VLAN has been renamed to
OFPAT_SET_VLAN_VID.