rio: add ^B control sequence to move cursor to output point

sys/include/keyboard.h

@@ -45,6 +45,8 @@ enum {
 	Kscrollonedown=	KF|0x21,
 
 	Ksoh=	0x01,
+	Kstx=	0x02,
+	Ketx=	0x03,
 	Keof=	0x04,
 	Kenq=	0x05,
 	Kack=	0x06,

sys/man/1/rio

@@ -391,7 +391,8 @@ for the preceding string (see
 .PP
 Typing a left or right arrow moves the cursor one character in that direction.
 Typing an SOH character (control-A) moves the cursor to the beginning of the
-current line; an ENQ character (control-E) moves to the end.
+current line; an ENQ character (control-E) moves to the end. The STX character
+(control-B) moves the cursor to the output point.
 .PP
 Text may be moved vertically within the window.
 A scroll bar on the left of the window shows in its clear portion what fragment of the

sys/src/cmd/rio/wind.c

@@ -691,6 +691,10 @@ wkeyctl(Window *w, Rune r)
 			wsetselect(w, q0, q0);
 			wshow(w, w->q0);
 			return;
+		case Kstx:	/* ^B: output point */
+			wsetselect(w, w->qh, w->qh);
+			wshow(w, w->q0);
+			return;
 		}
 	if(w->rawing && (w->q0==w->nr || w->mouseopen)){
 		waddraw(w, &r, 1);