ignore C1 control characters in UTF-8 mode

Ignore processing and printing C1 control characters in UTF-8 mode.
These are in the range: 0x80 - 0x9f.

By default in st the mode is set to UTF-8.

This matches more the behaviour of xterm with the options -u8 or +u8 also.
Also see the xterm resource "allowC1Printable".

Let me know if this breaks something, in most cases I don't think so.

As usual a very good reference is:
https://invisible-island.net/xterm/ctlseqs/ctlseqs.html

st.c

@@ -2422,6 +2422,9 @@ check_control_code:
 	 * they must not cause conflicts with sequences.
 	 */
 	if (control) {
+		/* in UTF-8 mode ignore handling C1 control characters */
+		if (IS_SET(MODE_UTF8) && ISCONTROLC1(u))
+			return;
 		tcontrolcode(u);
 		/*
 		 * control codes are not shown ever