ofproto: Add 'name' field to struct ofproto and use hmap instead of shash.

ofproto: Add 'name' field to struct ofproto and use hmap instead of shash.

It's slightly inconvenient to call into dpif_name() just to get the name
of an ofproto.  Furthermore, we're already keeping a copy of the ofproto's
name around, in the 'name' field of its shash_node.  It seems easier all
around if we just keep the name right in the struct ofproto and use an
hmap instead of a shash.