When using the --listen-on option have kitty @ automatically connect to the socket even when --to is not specified

The listen on value is set in the env of the child processes, so kitty @
can read it when run from inside kitty.

kitty/cli.py

@@ -109,11 +109,14 @@ regardless of this option.
 --listen-on
 Tell kitty to listen on the specified address for control
 messages. For example, :option:`{appname} --listen-on`=unix:/tmp/mykitty or
-:option:`{appname} --listen-on`=tcp:localhost:12345. On Linux systems, you can also use abstract
-UNIX sockets, not associated with a file, like this: :option:`{appname} --listen-on`=unix:@mykitty.
-To control kitty, you can send it commands with :italic:`kitty @` using the :option:`kitty @ --to` option
-to specify this address. Note that this option will be ignored, unless you set
-:opt:`allow_remote_control` to yes in :file:`kitty.conf`.
+:option:`{appname} --listen-on`=tcp:localhost:12345. On Linux systems, you can
+also use abstract UNIX sockets, not associated with a file, like this:
+:option:`{appname} --listen-on`=unix:@mykitty.  To control kitty, you can send
+it commands with :italic:`kitty @` using the :option:`kitty @ --to` option to
+specify this address. This option will be ignored, unless you set
+:opt:`allow_remote_control` to yes in :file:`kitty.conf`. Note that if you run
+:italic:`kitty @` within a kitty window, there is no need to specify the :italic:`--to`
+option as it is read automatically from the environment.
 
 
 # Debugging options

kitty/main.py

@@ -203,10 +203,12 @@ def macos_cmdline():
     return ans
 
 
-def setup_environment(opts):
+def setup_environment(opts, args):
     extra_env = opts.env.copy()
     if opts.editor != '.':
         os.environ['EDITOR'] = opts.editor
+    if args.listen_on:
+        os.environ['KITTY_LISTEN_ON'] = args.listen_on
     set_default_env(extra_env)
 
 
@@ -264,7 +266,7 @@ def _main():
             return
     init_glfw(args.debug_keyboard)  # needed for parsing native keysyms
     opts = create_opts(args)
-    setup_environment(opts)
+    setup_environment(opts, args)
     try:
         with setup_profiling(args):
             # Avoid needing to launch threads to reap zombies

kitty/remote_control.py

@@ -3,6 +3,7 @@
 # License: GPL v3 Copyright: 2018, Kovid Goyal <kovid at kovidgoyal.net>
 
 import json
+import os
 import re
 import sys
 import types
@@ -164,6 +165,8 @@ def main(args):
     else:
         no_response = func.no_response
     send['no_response'] = no_response
+    if not global_opts.to and 'KITTY_LISTEN_ON' in os.environ:
+        global_opts.to = os.environ['KITTY_LISTEN_ON']
     response = do_io(global_opts.to, send, no_response)
     if no_response:
         return