* ./src/misc/variables.c: callback loops are now detected; this means you
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.