Fix subshell leak for 4 special variables (re: bd3e2a80)

The following special variables leaked out of a subshell:
$_, ${.sh.name}, ${.sh.level}, ${.sh.subscript}.
This was due to a faulty optimisation in sh_assignok().
bd3e2a80 fixed that in part, this fixes the rest.

src/cmd/ksh93/sh/subshell.c:
- Simplify sh_assignok() by removing special-casing for these four
  special variables. The 'add' param reverts to a simple boolean.
- The test for a ${ subshare; } was actually wrong. sp->subshare is
  a saved backup value. We must test shp->subshare. (re: a9de50bf)

src/cmd/ksh93/bltins/typeset.c:
- setall(), unall(): Update sh_assignok() calls.

src/cmd/ksh93/tests/variables.sh:
- Regress-test subshell leaks for all special variables.

Closes: #122

src/cmd/ksh93/sh/subshell.c

@@ -233,10 +233,6 @@ int nv_subsaved(register Namval_t *np)
  *
  * add == 0:    Move the node pointer from the parent shell to the current virtual subshell.
  * add == 1:    Create a copy of the node pointer in the current virtual subshell.
- * add == 2:    This will create a copy of the node pointer like 1, but it will disable the
- *              optimization for ${.sh.level}.
- * add == 3:    This is like 1, but it will never skip the following variables:
- *              ${.sh.level}, $_, ${.sh.subscript} and ${.sh.name}.
  */
 Namval_t *sh_assignok(register Namval_t *np,int add)
 {
@@ -248,11 +244,8 @@ Namval_t *sh_assignok(register Namval_t *np,int add)
 	Namval_t		*mpnext;
 	Namarr_t		*ap;
 	unsigned int		save;
-	/* don't bother to save if in a ${ subshare; } */
-	if(sp->subshare)
-		return(np);
-	/* don't bother with this */
-	if(!sp->shpwd || (add != 3 && ((add != 2 && np==SH_LEVELNOD) || np==L_ARGNOD || np==SH_SUBSCRNOD || np==SH_NAMENOD)))
+	/* don't save if told not to (see nv_restore()) or if we're in a ${ subshare; } */
+	if(!sp->shpwd || shp->subshare)
 		return(np);
 	if((ap=nv_arrayptr(np)) && (mp=nv_opensub(np)))
 	{