Disable CSI u inside Midnight Commander for now

Using

    SHELL=$(command -v fish) mc

Midnight Commander will spawn a fish child with

    "function fish_prompt;"
    "echo \"$PWD\">&%d; fish_prompt_mc; kill -STOP %%self; end\n",

So fish_prompt will SIGSTOP itself using an uncatchable signal.

On ctrl-o, mc will send SIGCONT to give back control to the shell.
Another ctrl-o will be intercepted by mc to put the shell back to sleep.

Since mc wants to intercept at least ctrl-o -- also while fish is in control
-- we can't use the CSI u encoding until mc either understands that, or uses
a different way of passing control between mc and fish.

Let's disable it for now.

Note that mc still uses %self but we've added a feature flag
to disable that.  So if you use "set fish_features all"
you'll want to add a " no-remove-percent-self". A patch
to make mc use $fish_pid has been submitted upstream at
https://lists.midnight-commander.org/pipermail/mc-devel/2024-July/011226.html.

Closes #10640

src/input_common.rs

@@ -434,8 +434,12 @@ pub fn update_wait_on_sequence_key_ms(vars: &EnvStack) {
     MainThread::new(RefCell::new(None));
 
 pub(crate) static IS_TMUX: RelaxedAtomicBool = RelaxedAtomicBool::new(false);
+pub(crate) static IN_MIDNIGHT_COMMANDER: RelaxedAtomicBool = RelaxedAtomicBool::new(false);
 
 pub fn terminal_protocols_enable_ifn() {
+    if IN_MIDNIGHT_COMMANDER.load() {
+        return;
+    }
     let mut term_protocols = TERMINAL_PROTOCOLS.get().borrow_mut();
     if term_protocols.is_some() {
         return;

src/reader.rs

@@ -70,6 +70,7 @@
     SearchType,
 };
 use crate::input::init_input;
+use crate::input_common::IN_MIDNIGHT_COMMANDER;
 use crate::input_common::{
     terminal_protocols_disable_ifn, terminal_protocols_enable_ifn, CharEvent, CharInputStyle,
     InputData, ReadlineCmd, IS_TMUX,
@@ -3847,6 +3848,7 @@ fn reader_interactive_init(parser: &Parser) {
         .set_one(L!("_"), EnvMode::GLOBAL, L!("fish").to_owned());
 
     IS_TMUX.store(parser.vars().get_unless_empty(L!("TMUX")).is_some());
+    IN_MIDNIGHT_COMMANDER.store(parser.vars().get_unless_empty(L!("MC_TMPDIR")).is_some());
 }
 
 /// Destroy data for interactive use.