ofp-errors: Implement OpenFlow 1.2+ experimenter error codes.

OpenFlow 1.2 standardized experimenter error codes in a way different from
the Nicira extension.  This commit implements the OpenFlow 1.2+ version.

This commit also makes it easy to add error codes for new experimenter IDs
by adding new *_VENDOR_ID definitions to openflow-common.h.

Signed-off-by: Ben Pfaff <blp@nicira.com>
ofp-errors: New error code ONFBIC_DUP_INSTRUCTION.

This is a prototype of OpenFlow enhancement proposal EXT-260 "Add error
code for duplicate instruction."  It uses the error code proposed there.

Signed-off-by: Ben Pfaff <blp@nicira.com>