Support of Perforce Brokers

Modification of Perforce client to support Perforce brokers. First check if the
P4 Info has a Broker address defined .. if so use the broker, otherwise (as
before) check the Server address.

Testing Done:
Been using this mod for over a year at Cadence on RBTools 0.5.1

Reviewed at https://reviews.reviewboard.org/r/5062/

Fixes bug 2568.