ofp-util: Abstract flow_mod OFPFF_* flags.

ofp-util: Abstract flow_mod OFPFF_* flags.

The OFPFF_* flags used in flow_mods are just confusing enough that it
seems worthwhile to try to abstract them out.  In particular:

    * OFPFF_EMERG was introduced in OF1.0, deleted in OF1.1, and then
      its bit was reused for a different purpose in OF1.2.

    * OFPFF_RESET_COUNTS was introduced in OF1.2 but the semantics that it
      specifies are implied by "add" commands in earlier versions, so
      proper translation requires the OpenFlow version number and flow_mod
      command.

This commit does the abstraction.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>