libopenflow: Fix ofp_phy_port _validate()

This is a regression caused by a recent improvement.
libopenflow: Use more symbolic constants

Many things were using hardcoded literals.
libopenflow: Minor cleanup
libopenflow: Better default for port_stats_request

The port was defaulting to 0, but should be OFPP_NONE.
libopenflow: Improve adding of port_stats

port_stats objects can be added as an easy way of summing the counters.
Previously, this lost the port_no value.  Now, it is retained when it
is the same in both the left and right operand.