Disable the SUSP character

This makes binding \cz possible.

We already ignore the SIGTSTP signal it sends, so until now it was useless.

(also STOP and START for good measure, but since we disable flow
control in fish anyway these already shouldn't have been sent)

Fixes #7152

CHANGELOG.rst

@@ -78,6 +78,7 @@ Interactive improvements
 -  When pressing Tab, fish displays ambiguous completions even when they
    have a common prefix, without the user having to press Tab again
    (#6924).
+-  Control-z is now available for binding (#7152).
 
 
 New or improved bindings

src/reader.cpp

@@ -649,6 +649,20 @@ static void term_fix_modes(struct termios *modes) {
     modes->c_lflag &= ~IEXTEN;  // turn off handling of discard and lnext characters
     modes->c_lflag &= ~IEXTEN;  // turn off handling of discard and lnext characters
     modes->c_oflag |= OPOST;  // turn on "implementation-defined post processing" - this often changes how line breaks work.
+
+    // Disable flow control in the shell. We don't want to be stopped.
+    modes->c_iflag &= ~IXON;
+    modes->c_iflag &= ~IXOFF;
+
+    modes->c_cc[VMIN] = 1;
+    modes->c_cc[VTIME] = 0;
+
+    // We ignore these anyway, so there is no need to sacrifice a character.
+    modes->c_cc[VSUSP] = '\0';
+
+    // (these two are already disabled because of IXON/IXOFF)
+    modes->c_cc[VSTOP] = '\0';
+    modes->c_cc[VSTART] = '\0';
 }
 
 /// Tracks a currently pending exit. This may be manipulated from a signal handler.
@@ -1127,12 +1141,6 @@ void reader_init() {
     std::memcpy(&shell_modes, &terminal_mode_on_startup, sizeof shell_modes);
 
     term_fix_modes(&shell_modes);
-    // Disable flow control in the shell. We don't want to be stopped.
-    shell_modes.c_iflag &= ~IXON;
-    shell_modes.c_iflag &= ~IXOFF;
-
-    shell_modes.c_cc[VMIN] = 1;
-    shell_modes.c_cc[VTIME] = 0;
 
     // We do this not because we actually need the window size but for its side-effect of correctly
     // setting the COLUMNS and LINES env vars.