add RTP proxy mode for ip.access

Up until now, we only supported direct RTP streams between ip.access BTS.
With this commit, the user can specify '-P' to the command line to enable
a RTP/RTCP proxy inside OpenBSC.  The nanoBTS will then send all their voice
data to OpenBSC, which will relay it to the respective destination BTS (which
can be the same BTS).

The default behaviour remains unchanged.  Without '-P' on the command line,
RTP/RTCP is exchanged directly.
diff --git a/openbsc/src/bsc_hack.c b/openbsc/src/bsc_hack.c
index 1bfb706..ee8bd1f 100644
--- a/openbsc/src/bsc_hack.c
+++ b/openbsc/src/bsc_hack.c
@@ -64,6 +64,7 @@
 static enum gsm_bts_type BTS_TYPE = GSM_BTS_TYPE_BS11;
 static enum gsm_band BAND = GSM_BAND_900;
 static const char *database_name = "hlr.sqlite3";
+extern int ipacc_rtp_direct;
 
 struct nano_bts_id {
 	struct llist_head entry;
@@ -1113,10 +1114,11 @@
 			{"bts-id", 1, 0, 'i'},
 			{"tsc", 1, 0, 'S'},
 			{"bsic", 1, 0, 'B'},
+			{"rtp-proxy", 0, 0, 'P'},
 			{0, 0, 0, 0}
 		};
 
-		c = getopt_long(argc, argv, "hc:n:d:sar:p:f:t:C:RL:l:Tb:i:S:B:",
+		c = getopt_long(argc, argv, "hc:n:d:sar:p:f:t:C:RL:l:Tb:i:S:B:P",
 				long_options, &option_index);
 		if (c == -1)
 			break;
@@ -1187,6 +1189,9 @@
 		case 'B':
 			BSIC = atoi(optarg);
 			break;
+		case 'P':
+			ipacc_rtp_direct = 0;
+			break;
 		}
 		default:
 			/* ignore */