Changed default PROXY_PORT value to be whatever numeric portion of
the broker port is, plus one. So 1667 by default.
The immediate reason for this change is to simplify testing, since
the old default of just using P4PORT didn't allow easy testing of the
p4p on the same machine as p4d. That made sense insomuch as typically
a p4p runs on different machine than the p4d/p4broker stack. But this
new value makes as much sense, and is more flexible for testing.