can now use var_* functions from within your callback, they can even
    trigger callback themselves, to any level. The main restriction is that
    you shouldn't meddle with a variable that is already being triggered by
    the current thread (other threads will just wait).
  * ./src/misc/objects.c: fixed a deadlock in the "tree" command.
  * ./modules/misc/testsuite/test4.c: added a "callback-test" command to the
    rc interface to test callback loop detection and concurrent triggers.