Add network QoS support for PCP to iscsi target.

Mak the Ethernet PCP codepoint configurable
for L2 local traffic, to allow lower latency for
iSCSI block IO. This addresses the target
side only.

Reviewed by: mav, trasz, bcr
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26740