* debug.h (ModifyHandle): Define new macro.

* debug.h (ModifyHandle): Define new macro.
(modify_handle): Declare new function.
* debug.cc (modify_handle): Define new function.
* fhandler.h (fhandler_base::fork_fixup): Change return value from void to
bool.
* fhandler.cc (fhandler_base::fork_fixup): Return true if fork fixup has been
done.
* pipe.cc (fhandler_pipe::set_close_on_exec): Set inheritance of protected
handle via ModifyHandle if DEBUGGING.
(fhandler_pipe::fixup_after_fork): Protect guard handle if fork fixup has been
done.