ofproto: Check overlap, emerg flow cache, and error code sync (OpenFlow 0.9)

ofproto: Check overlap, emerg flow cache, and error code sync (OpenFlow 0.9)

This commit adds (some) support for a couple new OpenFlow 0.9 features:

    - The OFPFF_CHECK_OVERLAP flag in Flow Mod messages allows the
      controller to prevent flows that would conflict at the same
      priority.

    - An emergency flow cache that contains a small flow table that is
      used if the switch loses connectivity with the controller.  I
      believe the design has fundamental flaws and looks likely to be
      retired.  If a controller attempts to add a flow to the emergency
      flow cache, OVS always responds that the tables are full.

The OpenFlow 0.9 error codes are also sync'd in the commit.

NOTE: OVS at this point is not wire-compatible with OpenFlow 0.9 until the
final commit in this OpenFlow 0.9 set.