release number bumped to 858 by export.pike
release number bumped to 859 by export.pike
release number bumped to 860 by export.pike
release number bumped to 861 by export.pike
Backend: Fixed potential double free on exit.

The code that freed registered fd boxes on termination of a backend
did not invalidate the reference, this may have caused a reference
underflow for fds that still were registered in the now dead backend.

We now use the proper unhook code to de-register the fd boxes.
release number bumped to 862 by export.pike
release number bumped to 863 by export.pike