Deprecate g_atexit

This function was just not a good idea to begin with.
Its documentation gives plenty of reason not to use it.

glib/gutils.c

@@ -251,6 +251,9 @@ g_memmove (gpointer      dest,
  * As can be seen from the above, for portability it's best to avoid
  * calling g_atexit() (or atexit()) except in the main executable of a
  * program.
+ *
+ * Deprecated:2.32: It is best to avoid g_atexit(), for the reasons
+ *     outlined above.
  */
 void
 g_atexit (GVoidFunc func)