Add SI and SO control codes

SI and SO allows change the G0 and G1 selection. This implementation is not
full vt100 compatible, but it is complatible with linux virtual terminal
implementation. For full vt100 compatibility we need remake a lot of stuff
relate to the different charmaps.
---
 st.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

st.c

@@ -1814,9 +1814,12 @@ tputc(char *c, int len) {
 		csireset();
 		term.esc = ESC_START;
 		return;
-	case '\016':	/* XXX: SO */
-	case '\017':	/* XXX: SI */
+	case '\016':	/* SO */
+		term.c.attr.mode |= ATTR_GFX;
 		break;
+	case '\017':	/* SI */
+		term.c.attr.mode &= ~ATTR_GFX;
+		return;
 	case '\032':	/* SUB */
 	case '\030':	/* CAN */
 		csireset();