rename ipacc_connect_proxy_bind() to rsl_ipacc_mdcx_to_rtpsock()

Our RTP sockets are no longer just for the proxy mode, so having
"proxy" in the function name is really misleading.
diff --git a/openbsc/include/openbsc/abis_rsl.h b/openbsc/include/openbsc/abis_rsl.h
index f0a5619..797b2f3 100644
--- a/openbsc/include/openbsc/abis_rsl.h
+++ b/openbsc/include/openbsc/abis_rsl.h
@@ -538,6 +538,7 @@
 int rsl_ipacc_crcx(struct gsm_lchan *lchan);
 int rsl_ipacc_mdcx(struct gsm_lchan *lchan, u_int32_t ip,
 		   u_int16_t port, u_int8_t rtp_payload2);
+int rsl_ipacc_mdcx_to_rtpsock(struct gsm_lchan *lchan);
 int rsl_ipacc_pdch_activate(struct gsm_lchan *lchan);
 
 int abis_rsl_rcvmsg(struct msgb *msg);