Make C-g' after M-x' not give a backtrace unless required

* src/eval.c (signal_quit_p): New function.
(maybe_call_debugger): React to all `quit' signals (bug#49675).

* src/keyboard.c (cmd_error_internal, menu_item_eval_property_1):
Ditto.

src/eval.c

@@ -2026,6 +2026,18 @@ skip_debugger (Lisp_Object conditions, Lisp_Object data)
   return 0;
 }
 
+/* Say whether SIGNAL is a `quit' symbol (or inherits from it).  */
+bool
+signal_quit_p (Lisp_Object signal)
+{
+  Lisp_Object list;
+
+  return EQ (signal, Qquit)
+    || (Fsymbolp (signal)
+	&& CONSP (list = Fget (signal, Qerror_conditions))
+	&& Fmemq (Qquit, list));
+}
+
 /* Call the debugger if calling it is currently enabled for CONDITIONS.
    SIG and DATA describe the signal.  There are two ways to pass them:
     = SIG is the error symbol, and DATA is the rest of the data.
@@ -2044,7 +2056,7 @@ maybe_call_debugger (Lisp_Object conditions, Lisp_Object sig, Lisp_Object data)
       ! input_blocked_p ()
       && NILP (Vinhibit_debugger)
       /* Does user want to enter debugger for this kind of error?  */
-      && (EQ (sig, Qquit)
+      && (signal_quit_p (sig)
 	  ? debug_on_quit
 	  : wants_debugger (Vdebug_on_error, conditions))
       && ! skip_debugger (conditions, combined_data)

src/keyboard.c

@@ -985,7 +985,7 @@ cmd_error_internal (Lisp_Object data, const char *context)
 {
   /* The immediate context is not interesting for Quits,
      since they are asynchronous.  */
-  if (EQ (XCAR (data), Qquit))
+  if (signal_quit_p (XCAR (data)))
     Vsignaling_function = Qnil;
 
   Vquit_flag = Qnil;
@@ -7634,7 +7634,7 @@ menu_item_eval_property_1 (Lisp_Object arg)
 {
   /* If we got a quit from within the menu computation,
      quit all the way out of it.  This takes care of C-] in the debugger.  */
-  if (CONSP (arg) && EQ (XCAR (arg), Qquit))
+  if (CONSP (arg) && signal_quit_p (XCAR (arg)))
     quit ();
 
   return Qnil;

src/lisp.h

@@ -4116,6 +4116,7 @@ extern Lisp_Object Vautoload_queue;
 extern Lisp_Object Vrun_hooks;
 extern Lisp_Object Vsignaling_function;
 extern Lisp_Object inhibit_lisp_code;
+extern bool signal_quit_p (Lisp_Object);
 
 /* To run a normal hook, use the appropriate function from the list below.
    The calling convention: