sFlow export LAG, PORTNAME and OPENFLOW structures

This patch extends the sFlow export to include the standard sFlow LACP structure when sending sFlow counters for ports involved in bundles. This information is useful for monitoring LAGs, and in some cases it is essential for correctly rolling up end-to-end traffic matrix data at an sFlow collector:

http://sflow.org/sflow_lag.txt

This patch also adds the standard sFlow structure to identify ports by their name, OpenFlow port-number and OpenFlow DPID.  This greatly simplifies the problem of reconciling the IP/ifIndex world of sFlow (and SNMP) with the DPID/port-number world of OpenFlow.  It therefore streamlines some of the steps required to use sFlow data to drive adaptive-control applications:

http://sflow.org/sflow_openflow.txt

The standard sFlow templates for exporting tunnel encapsulations are included in the sFlow library,  but these are not populated yet.  There is some work to do to decide on the best way to include those fields,  so it was decided to leave that to a future patch.

This patch can be considered a follow-up to this discussion thread from July 2014:
http://openvswitch.org/pipermail/dev/2014-July/042470.html

Signed-off-by: Neil McKee<neil.mckee@inmon.com>