OpenFlow: add vendor name printing

The new function goes into print-openflow.c as vendor name decoding is
the same in all versions of OpenFlow (although in 1.0 it is "vendor"
and in subsequent versions it is "experimenter"). The mapping is from:

https://rs.opennetworking.org/wiki/display/PUBLIC/ONF+Registry
OpenFlow: improve vendor message decoding

Introduce a new function that tries to dispatch an OFPT_VENDOR message
to a vendor-specific printer function. Add such a function for Big
Switch Networks vendor with about as little decoding as necessary to
cover the existing 7050Q OF1.0 capture.