gss: add gss_mg_name_to_oid internal API

Add a new function for future internal use, gss_mg_name_to_oid(), which takes
either a dot-separated OID or a "short" mechanism name (such as
"sanon-x25519"), and returns a mechanism OID.

lib/gssapi/version-script.map

@@ -69,6 +69,7 @@ HEIMDAL_GSS_2.0 {
 		gss_krb5_set_allowable_enctypes;
 		gss_localname;
 		gss_mg_collect_error;
+		gss_mg_name_to_oid;
 		gss_oid_equal;
 		gss_oid_to_str;
 		gss_pname_to_uid;