conntrack: Add support for NAT.

Extend OVS conntrack interface to cover NAT.  New nested NAT action
may be included with a CT action.  A bare NAT action only mangles
existing connections.  If a NAT action with src or dst range attribute
is included, new (non-committed) connections are mangled according to
the NAT attributes.

This work extends on a branch by Thomas Graf at
https://github.com/tgraf/ovs/tree/nat.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>

lib/flow.c

@@ -872,6 +872,10 @@ const char *ct_state_to_string(uint32_t state)
         return "rel";
     case CS_INVALID:
         return "inv";
+    case CS_SRC_NAT:
+        return "snat";
+    case CS_DST_NAT:
+        return "dnat";
     default:
         return NULL;
     }