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

kittens/ssh/main.py

@@ -39,7 +39,10 @@ from kitty.utils import (
     set_echo as turn_off_echo
 )
 
-from ..tui.operations import restore_colors, save_colors
+from ..tui.operations import (
+    RESTORE_PRIVATE_MODE_VALUES, SAVE_PRIVATE_MODE_VALUES, Mode,
+    restore_colors, save_colors, set_mode
+)
 from ..tui.utils import kitty_opts, running_in_tmux
 from .config import init_config
 from .copy import CopyInstruction
@@ -547,10 +550,13 @@ def connection_sharing_args(kitty_pid: int) -> List[str]:
 def restore_terminal_state() -> Iterator[bool]:
     with open(os.ctermid()) as f:
         val = termios.tcgetattr(f.fileno())
+        print(end=SAVE_PRIVATE_MODE_VALUES)
+        print(end=set_mode(Mode.HANDLE_TERMIOS_SIGNALS), flush=True)
         try:
             yield bool(val[3] & termios.ECHO)
         finally:
             termios.tcsetattr(f.fileno(), termios.TCSAFLUSH, val)
+            print(end=RESTORE_PRIVATE_MODE_VALUES, flush=True)
 
 
 def dcs_to_kitty(payload: Union[bytes, str], type: str = 'ssh') -> bytes: