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/gssapi_mech.h

@@ -701,6 +701,9 @@ gss_cred_id_t
 _gss_mg_find_mech_cred(gss_const_cred_id_t cred_handle,
 		       gss_const_OID mech_type);
 
+GSSAPI_LIB_FUNCTION gss_const_OID
+GSSAPI_CALLCONV gss_mg_name_to_oid(const char *);
+
 #include <krb5.h>
 
 /*