Handle forward-delete key in emacs and vi editors

On every modern system, the forward-delete key on PC/Mac keyboards
generates the VT220 sequence ESC [ 3 ~. Every other shell with an
editor handles this now, ksh93 seems to be the last not to.

src/cmd/ksh93/edit/emacs.c: escape():
- Handle the ^[[3 as part of normal escape processing, then read an
  extra character to check for the final '~'. If detected, insert
  an ERASECHAR key event.

src/cmd/ksh93/edit/vi.c: mvcursor():
- Replace the ^[[3~ sequence by an 'x' command. We have to
  special-case its processing, because vi mode parses numbers as
  repetition operators. The escape sequence contains a number,
  making it incompatible with normal command handling. This means
  number repetitions don't work with the forward-delete key. If
  that annoys anyone enough to fix it, a patch would be welcome.
  For now, it will do to make the forward-delete key stop
  exhibiting bizarre behaviour (beep + change case + move forward).

src/cmd/ksh93/sh.1
- Copy-edit emacs documentation for VT220-style sequences; map them
  to their actual key, otherwise it's meaningless to the reader.
- Document the new forward-delete key behaviour for emacs mode.
- Leave the forward-delete key for vi mode undocumented for now, as
  repetitions don't work, so it doesn't really match the vi canon.
  (OTOH, it doesn't work in vim, either...)

src/cmd/ksh93/edit/emacs.c

@@ -1109,6 +1109,19 @@ static int escape(register Emacs_t* ep,register genchar *out,int count)
 			    case 'Y':
 				ed_ungetchar(ep->ed,cntl('E'));
 				return(-1);
+			    case '3':
+				if(ed_getchar(ep->ed,1)=='~')
+				{	/*
+					 * VT220 forward-delete key.
+					 * Since ERASECHAR and EOFCHAR are usually both mapped to ^D, we
+					 * should only issue ERASECHAR if there is something to delete,
+					 * otherwise forward-delete on empty line will terminate the shell.
+					 */
+					if(cur < eol)
+						ed_ungetchar(ep->ed,ERASECHAR);
+					return(-1);
+				}
+				/* FALLTHROUGH */
 			    default:
 				ed_ungetchar(ep->ed,i);
 			}

src/cmd/ksh93/edit/vi.c

@@ -1606,6 +1606,7 @@ static int mvcursor(register Vi_t* vp,register int motion)
 	register int tcur_virt;
 	register int incr = -1;
 	register int bound = 0;
+	int c;
 
 	switch(motion)
 	{
@@ -1632,7 +1633,14 @@ static int mvcursor(register Vi_t* vp,register int motion)
 		break;
 
 	case '[':
-		switch(motion=getcount(vp,ed_getchar(vp->ed,-1)))
+		c = ed_getchar(vp->ed,-1);
+		if(c=='3' && ed_getchar(vp->ed,-1)=='~')
+		{
+			/* VT220 forward-delete key */
+			ed_ungetchar(vp->ed,'x');
+			return(1);
+		}
+		switch(motion=getcount(vp,c))
 		{
 		    case 'A':
 #if SHOPT_EDPREDICT