VERSA: Implement tab and backspace.

@ uses tabs when formatting /C cross-reference pages.

src/dcp/versa.211

@@ -4578,7 +4578,30 @@ normal:	skipn outena
 	return
 
 backsp:
-tab:	aos (p)
+tab:	skipn outena		;do nothing if output disabled
+	 jrst bspfin
+	push p,x		;save char
+	move x,xgp.x		;save original pos
+	push p,x
+	movei x,40		;output space
+	call normal
+	 jfcl
+	move z,xgp.x		;compute width of space (including seper)
+	pop p,x			;restore original pos
+	sub z,x
+	pop p,y			;are we doing a backspace?
+	cain y,^H
+	 jrst bspbsp
+	imuli z,8		;compute width of tab
+	sub x,lftmar		;how many tabs are we from the margin?
+	idiv x,z
+	aos x			;move right to next tab stop
+	imul x,z
+	add x,lftmar
+	skipa
+bspbsp:	 sub x,z		;move left by one space
+	movem x,xgp.x
+bspfin:	aos (p)
 	return
 
 linefd:	jrst linfin