refactor(channel): eliminate special case in on_proc_exit() #32485

Problem:
on_proc_exit() has a special-case that assumes that the UI client will
never spawn more than 1 child process.

Solution:
If the Nvim server exits, the stream EOF will trigger `rpc_close()` in
the UI client, so we don't need the special case in `on_proc_exit`.
Pass `Channel.exit_status` from `rpc_close()` so that the correct exit
code is reflected.

src/nvim/channel.h

@@ -40,7 +40,7 @@ struct Channel {
   CallbackReader on_data;
   CallbackReader on_stderr;
   Callback on_exit;
-  int exit_status;
+  int exit_status;  ///< Process exit-code (if the channel wraps a process).
 
   bool callback_busy;
   bool callback_scheduled;