Add DSCP support for network QoS to iscsi initiator.

[rscheff]

Summary

Similar to D26385, the DSCP codepoing should also
be configurable for the traffic in the direction
from the initiator to the target, such that writes
and any requests are also treated in the appropriate
QoS class.

Test Plan

Verify that the initial TCP packets sent towards
a iscsi target, and all subsequent packets in that
direction, are marked with the configured DSCP value.

[rscheff]

• add dscp to man page, and keep equal sign between keywords
• output dscp of target only if configured