Fix techo handling of control and multibyte characters.

techo compares signed char to '\x20'. Any character with code less then
'\x20' is treated as control character.  This way characters with MSB
set to 1 are considered control characters too.

Also this patch makes techo display DEL character as ^?.

To reprocuce the bug, enable echo mode using printf '\e[12l',
then type DEL character or any non-ASCII character.

st.c

@@ -2308,9 +2308,9 @@ techo(char *buf, int len) {
 	for(; len > 0; buf++, len--) {
 		char c = *buf;
 
-		if(c < '\x20') { /* control code */
+		if(c < 0x20 || c == 0177) { /* control code */
 			if(c != '\n' && c != '\r' && c != '\t') {
-				c |= '\x40';
+				c ^= '\x40';
 				tputc("^", 1);
 			}
 			tputc(&c, 1);