of_01: Fix removals from PortCollection

Previously, _forget() took a port_no, but was being passed an entire
ofp_phy_port.  This is unlike _update() which actually needed the entire
ofp_phy_port.  For consistency's sake, they now both take ofp_phy_port.
This could be re-evaluated if there's ever another caller of _forget().

While _forget() and _update() took different parameter types, they were
both *called* the same way.  This led to a situation where ports were not
actually masked/removed.  Fixed.  (I think Jamie probably ran into this
bug a while back but it never got tracked down.)

This also adds a bit of documentation for PortCollection.

Closes #113