rcfile: allow to rebind the Cancel function in the yesno menu

This fixes the first part of https://savannah.gnu.org/bugs/?54447.

doc/nano.texi

@@ -1441,6 +1441,9 @@ The 'search to replace' menu.
 @item replacewith
 The 'replace with' menu, which comes up after 'search to replace'.
 
+@item yesno
+The 'yesno' menu, where the Yes/No/All/Cancel question is asked.
+
 @item gotoline
 The 'goto line (and column)' menu.
 

doc/nanorc.5

@@ -792,6 +792,9 @@ The 'search to replace' menu.
 .B replacewith
 The 'replace with' menu, which comes up after 'search to replace'.
 .TP
+.B yesno
+The 'yesno' menu, where the Yes/No/All/Cancel question is asked.
+.TP
 .B gotoline
 The 'goto line (and column)' menu.
 .TP

src/global.c

@@ -1713,6 +1713,8 @@ int strtomenu(const char *input)
 		return MREPLACE;
 	else if (!strcasecmp(input, "replacewith"))
 		return MREPLACEWITH;
+	else if (!strcasecmp(input, "yesno"))
+		return MYESNO;
 	else if (!strcasecmp(input, "gotoline"))
 		return MGOTOLINE;
 	else if (!strcasecmp(input, "writeout"))

src/prompt.c

@@ -678,6 +678,8 @@ int do_yesno_prompt(bool all, const char *msg)
 		if (!ISSET(NO_HELP)) {
 			char shortstr[MAXCHARLEN + 2];
 				/* Temporary string for (translated) " Y", " N" and " A". */
+			const sc *cancelshortcut = first_sc_for(MYESNO, do_cancel);
+				/* The keystroke that is bound to the Cancel function. */
 
 			if (COLS < 32)
 				width = COLS / 2;
@@ -701,7 +703,7 @@ int do_yesno_prompt(bool all, const char *msg)
 			}
 
 			wmove(bottomwin, 2, width);
-			post_one_key("^C", _("Cancel"), width);
+			post_one_key(cancelshortcut->keystr, _("Cancel"), width);
 		}
 
 		/* Color the statusbar over its full width and display the question. */