Rename check_cancel_from_fish_signal to fish_is_unwinding_for_exit

"unwinding_for_exit" mixes up SIGHUP handling and also the exit builtin;
this is still pretty messy.

src/parse_execution.cpp

@@ -228,7 +228,7 @@ process_type_t parse_execution_context_t::process_type_for_command(
 maybe_t<end_execution_reason_t> parse_execution_context_t::check_end_execution() const {
     // If one of our jobs ended with SIGINT, we stop execution.
     // Likewise if fish itself got a SIGINT, or if something ran exit, etc.
-    if (cancel_signal || ctx.check_cancel() || check_cancel_from_fish_signal()) {
+    if (cancel_signal || ctx.check_cancel() || fish_is_unwinding_for_exit()) {
         return end_execution_reason_t::cancelled;
     }
     const auto &ld = parser->libdata();

src/proc.cpp

@@ -951,7 +951,7 @@ void job_t::continue_job(parser_t &parser) {
           parser.libdata().is_interactive ? L"INTERACTIVE" : L"NON-INTERACTIVE");
 
     // Wait for the status of our own job to change.
-    while (!check_cancel_from_fish_signal() && !is_stopped() && !is_completed()) {
+    while (!fish_is_unwinding_for_exit() && !is_stopped() && !is_completed()) {
         process_mark_finished_children(parser, true);
     }
     if (is_completed()) {

src/reader.cpp

@@ -963,7 +963,7 @@ static void term_steal() {
     termsize_container_t::shared().invalidate_tty();
 }
 
-bool check_cancel_from_fish_signal() {
+bool fish_is_unwinding_for_exit() {
     switch (s_exit_state) {
         case exit_state_t::none:
             // Cancel if we got SIGHUP.

src/reader.h

@@ -234,9 +234,8 @@ void reader_pop();
 /// The readers interrupt signal handler. Cancels all currently running blocks.
 void reader_handle_sigint();
 
-/// \return whether we should cancel fish script due to fish itself receiving a signal.
-/// TODO: this doesn't belong in reader.
-bool check_cancel_from_fish_signal();
+/// \return whether fish is currently unwinding the stack in preparation to exit.
+bool fish_is_unwinding_for_exit();
 
 /// Given a command line and an autosuggestion, return the string that gets shown to the user.
 /// Exposed for testing purposes only.