Add iscntrl_ascii()

It's too easy to miss the DEL character.

liberty.c

@@ -2672,6 +2672,12 @@ strncasecmp_ascii (const char *a, const char *b, size_t n)
 	return 0;
 }
 
+static bool
+iscntrl_ascii (int c)
+{
+	return (c >= 0 && c < 32) || c == 0x7f;
+}
+
 static bool
 isalpha_ascii (int c)
 {