Introduce ofputil_protocol, to abstract the protocol in use on a connection.

Introduce ofputil_protocol, to abstract the protocol in use on a connection.

Open vSwitch already handles a few different protocol variations, but it
does so in a nonuniform manner:

  - OpenFlow 1.0 and NXM flow formats are distinguished using the NXFF_*
    constant values from nicira-ext.h.

  - The "flow_mod_table_id" feature setting is maintained in ofproto as
    part of an OpenFlow connection's (ofconn's) state.

There's no way to easily communicate this state among components.  It's
not much of a problem yet, but as more protocol support is added it seems
better to have an abstract, uniform way to represent protocol versions and
variants.  This commit implements that by introducing a new type
"enum ofputil_protocol".  Each ofputil_protocol value represents a variant
of a protocol version.  Each value is a separate bit, so a single enum
can also represent a set of protocols, which is often useful as well.

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