Add DSCP support for network QoS to iscsi target.

In order to prioritize iSCSI traffic across a network,
DSCP can be used. In order not to rely on "ipfw setdscp"
or in-network reclassification, this adds the dscp value
directly to the portal group (where TCP sessions are accepted).

The incoming iSCSI session is first handled by ctld for any
CHAP authentication and the socket is then handed off to the
in-kernel iscsi driver without modification of the socket
parameters. Simply setting up the socket in ctld is sufficient
to keep sending outgoing iSCSI related traffic with the
configured DSCP value.

Reviewed by: mav, trasz
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26385