Simplify rfofmsg.cc functions

This patch series consolidates and simplifies a few of the functions in rfofmsg.cc.

Functionally, we see a change in the DC_DROP_ALL rule installation, which previously associated with buffer 0, but is now 0xffffffff (no associated buffer, we call this OFP_BUFFER_NONE).

There are also new functions to simplify OF1.0 IP mask creation based on CIDR or full IPv4 masks.