ssh kitten: Allow pressing Ctrl-C to abort ssh before the connection is completed

Fixing this involved adding a new mode to kitty where it handles
ctrl-c/z/q by sending signals to the tty foreground process group
instead of delegating to the kernel to do that. Since the pipe may be
full we have no way of knowing when the kernel will get around to
reading the signal byte. So send the signal ourselves.

Fixes #5271

kitty/window.py

@@ -857,6 +857,13 @@ class Window:
             '--ssh-connection-data', json.dumps(conn_data)
         )
 
+    def send_signal_for_key(self, key_num: int) -> bool:
+        try:
+            return self.child.send_signal_for_key(key_num)
+        except OSError as err:
+            log_error(f'Failed to send signal for key to child with err: {err}')
+            return False
+
     def focus_changed(self, focused: bool) -> None:
         if self.destroyed:
             return