OVSDriver: use IFF_RUNNING for link state

IFF_RUNNING includes both the local and remote sides, where IFF_UP is only the
local side. An interface cann have IFF_UP set without IFF_RUNNING.
OVSDriver: support PORT_DOWN config bit in port-mod

This bit is synchronized with the IFF_UP interface flag.
Merge into master from pull request #114:
support OFPPC_PORT_DOWN (https://github.com/floodlight/ivs/pull/114)