Fix portability problem in techo()

ISCONTROL chechks if a value is between 0 and 0x1f or
between 0x80 and 0x9f. Char signess depends of architecture
and compiler, so in some environment the second case is
always false (and wrong), Techo() calls ISCONTROL with a
char variable, whose type cannot be changed because tpuc()
expects a pointer to char, so the solution is to insert a
cast in the call to ISCONTROL.

st.c

@@ -2311,13 +2311,13 @@ techo(char *buf, int len) {
 	for(; len > 0; buf++, len--) {
 		char c = *buf;
 
-		if(ISCONTROL(c)) { /* control code */
+		if(ISCONTROL((uchar) c)) { /* control code */
 			if(c & 0x80) {
 				c &= 0x7f;
 				tputc("^", 1);
 				tputc("[", 1);
 			} else if(c != '\n' && c != '\r' && c != '\t') {
-				c ^= '\x40';
+				c ^= 0x40;
 				tputc("^", 1);
 			}
 			tputc(&c, 1);