Add a SIGINT handler that kills the process tree

We're not actually using this but it's for reference if we do end up
needing it.

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 ignore 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

@@ -196,6 +196,22 @@ async def _acquire_debug_lock(
         log.debug(f"TTY lock released, remote task: {task_name}:{uid}")
 
 
+def handler(signum, frame, *args):
+    """Specialized debugger compatible SIGINT handler.
+
+    In childred we always ignore to avoid deadlocks since cancellation
+    should always be managed by the parent supervising actor. The root
+    is always cancelled on ctrl-c.
+    """
+    if is_root_process():
+        tractor.current_actor().cancel_soon()
+    else:
+        print(
+            "tractor ignores SIGINT while in debug mode\n"
+            "If you have a special need for it please open an issue.\n"
+        )
+
+
 @tractor.context
 async def _hijack_stdin_for_child(