ofproto-dpif: Move process_special() to ofproto-dpif-xlate.c.

The action translation functions are the only ones that need
process_special().  Move that function closer to the callers, since a
future commit will use more xlate-related knowledge in process_special.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>