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/contrib/testconv/testconv_main.c b/openbsc/contrib/testconv/testconv_main.c
index 89dce1a..773be26 100644
--- a/openbsc/contrib/testconv/testconv_main.c
+++ b/openbsc/contrib/testconv/testconv_main.c
@@ -56,7 +56,7 @@
 	tcfg.endpoints = &endp;
 	tcfg.number_endpoints = 1;
 	endp.tcfg = &tcfg;
-	mgcp_free_endp(&endp);
+	mgcp_initialize_endp(&endp);
 
 	dst_end = &endp.bts_end;
 	src_end = &endp.net_end;