ipa: Only use one IPA number for Osmo extensions

Instead of using more numbers from the proto range we will
use the 0xee and then have a mini header with our new proto
id in there. For a start rename the use types to _OLD.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 046f38c..a0bebf1 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -958,7 +958,7 @@
 			goto exit2;
 			break;
 		}
-        } else if (parsed->ipa_proto == IPAC_PROTO_MGCP) {
+        } else if (parsed->ipa_proto == IPAC_PROTO_MGCP_OLD) {
                 bsc_mgcp_forward(bsc, msg);
                 goto exit2;
 	} else {