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_mgcp/mgcp_main.c b/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
index 8c3808a..16fb722 100644
--- a/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
+++ b/openbsc/src/osmo-bsc_mgcp/mgcp_main.c
@@ -181,7 +181,7 @@
 
 		/* is checking in_addr.s_addr == INADDR_LOOPBACK making it more secure? */
 		for (i = 1; i < reset_trunk->number_endpoints; ++i)
-			mgcp_free_endp(&reset_trunk->endpoints[i]);
+			mgcp_release_endp(&reset_trunk->endpoints[i]);
 	}
 
 	return 0;