xbuf: initial commit

This module provides an expandable buffer datastructure. The main use will be
efficient lists of small elements, like the upcoming internal action
representation. It includes Netlink-compatible attributes which can be nested.
OVSDriver: remove unused hash_mask creation
OVSDriver: translate actions to an internal form

There are a few reasons to do this:
 - Abstract out some differences between OF 1.0/1.3 (mostly set-field actions).
 - Create richer actions than in OpenFlow.
 - Speed up action translation in the upcall path.
xbuf: add compact operation

Use it in OVSDriver to slightly reduce memory usage.