patch 8.2.1335: CTRL-C in the GUI doesn't interrupt

Problem:    CTRL-C in the GUI doesn't interrupt. (Sergey Vlasov)
Solution:   Recognize "C" with CTRL modifier as CTRL-C. (issue #6565)
diff --git a/src/gui.c b/src/gui.c
index 4981d73..e474554 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5575,3 +5575,24 @@
     entered = FALSE;
 }
 #endif
+
+/*
+ * Check if "key" is to interrupt us.  Handles a key that has not had modifiers
+ * applied yet.
+ * Return the key with modifiers applied if so, NUL if not.
+ */
+    int
+check_for_interrupt(int key, int modifiers_arg)
+{
+    int modifiers = modifiers_arg;
+    int c = merge_modifyOtherKeys(key, &modifiers);
+
+    if ((c == Ctrl_C && ctrl_c_interrupts)
+	    || (intr_char != Ctrl_C && c == intr_char))
+    {
+	got_int = TRUE;
+	return c;
+    }
+    return NUL;
+}
+