mgcp: Change API to remove memory management from the name

Jacob pointed out that "free_endp" refers to the memory of
the endpoint being freed. What we want is actually a way to
release an endpoint (and the resource it allocated) or in
the case of the testcase/testapp initialize the data structure
correctly. Introduce two names for that.
diff --git a/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c b/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
index e13827b..4a0aaff 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_mgcp_utils.c
@@ -251,7 +251,7 @@
 
 	for (i = 1; i < nat->mgcp_cfg->trunk.number_endpoints; ++i){
 		bsc_mgcp_free_endpoint(nat, i);
-		mgcp_free_endp(&nat->mgcp_cfg->trunk.endpoints[i]);
+		mgcp_release_endp(&nat->mgcp_cfg->trunk.endpoints[i]);
 	}
 }
 
@@ -621,7 +621,7 @@
 	}
 
 	bsc_mgcp_free_endpoint(bsc->nat, ENDPOINT_NUMBER(endp));
-	mgcp_free_endp(endp);
+	mgcp_release_endp(endp);
 }
 
 /*
@@ -1065,6 +1065,6 @@
 			rate_ctr_inc(ctr);
 
 		bsc_mgcp_free_endpoint(bsc->nat, i);
-		mgcp_free_endp(&bsc->nat->mgcp_cfg->trunk.endpoints[i]);
+		mgcp_release_endp(&bsc->nat->mgcp_cfg->trunk.endpoints[i]);
 	}
 }