ofp-msgs: New approach to encoding and decoding OpenFlow headers.

ofp-msgs: New approach to encoding and decoding OpenFlow headers.

OpenFlow headers are not as uniform as they could be, with size, alignment,
and numbering changes from one version to another and across varieties
(e.g. ordinary messages vs. "stats" messages).  Until now the Open vSwitch
internal APIs haven't done a good job of abstracting those differences in
header formats.  This commit changes that; from this commit forward very
little code actually needs to understand the header format or numbering.
Instead, it can just encode or decode, or pull or put, the header using
a more abstract API using the ofpraw_, ofptype_, and other APIs in the
new ofp-msgs module.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Tested-by: Simon Horman <horms@verge.net.au>
Reviewed-by: Simon Horman <horms@verge.net.au>