Make SIGINT handler kill the process tree

The std lib's `pdb` internals override SIGINT handling whenever one
enters the debugger repl. Force a handler that kills the tree if SIGINT
is triggered from the root actor, otherwise igore it since supervised
children should be managed already. This resolves an issue with guest
mode where `pdb` causes SIGINTs to be swallowed resulting in the host
loop never terminating the process tree.

tractor/_debug.py

@@ -248,6 +248,7 @@ def _breakpoint(debug_func) -> Awaitable[None]:
             # may have the tty locked prior
             if _debug_lock.locked():  # root process already has it; ignore
                 return
+
             await _debug_lock.acquire()
             _pdb_release_hook = _debug_lock.release