cxgbetool(8): User interface to round-robin queue selection via COP.

Queue "roundrobin" in a COP rule means the driver should select queues
for new tids in a round-robin manner.

Reviewed by: jhb@
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D34922

(cherry picked from commit 811a82d53aaf905bebb523600d375a457069f022)