Remove a buggy optimization for variables in subshells

This bug was originally reported by @lijog in att/ast#7 and has been
reported again in #15. KSH does not save the state of a variable if it
is in a newer scope. This is because of an optimization in sh_assignok
first introduced in ksh93t+ 2010-05-24. Here is the code change in that
version:

                return(np);
        /* don't bother to save if in newer scope */
-       if(!(rp=shp->st.real_fun)  || !(dp=rp->sdict))
-               dp = sp->var;
-       if(np->nvenv && !nv_isattr(np,NV_MINIMAL|NV_EXPORT) && shp->last_root)
-               dp = shp->last_root;
-       if((mp=nv_search((char*)np,dp,HASH_BUCKET))!=np)
-       {
-               if(mp || !np->nvfun || np->nvfun->subshell>=sh.subshell)
-                       return(np);
-       }
+       if(sp->var!=shp->var_tree && shp->last_root==shp->var_tree)
+               return(np);
        if((ap=nv_arrayptr(np)) && (mp=nv_opensub(np)))
        {

This change was originally made to replace a buggier optimization.
However, the current optimization causes variables set in subshells
to wrongly affect the environment outside of the subshell, as the
variable does not get set back to its original value. This patch
simply removes the buggy optimization to fix this problem.

src/cmd/ksh93/sh/subshell.c:
 - Remove a buggy optimization that caused variables set in subshells
   to affect the environment outside of the subshell.

src/cmd/ksh93/tests/subshell.sh:
 - Add a regression test for setting variables in subshells. This
   test has to be run from the disk after being created with a here
   document because it always returns the expected result when run
   directly in the regression test script.

src/cmd/ksh93/tests/subshell.sh

@@ -726,5 +726,18 @@ check_hash_table()
 (hash cat)
 [[ $(hash) == "chmod=$(whence -p chmod)" ]] || err_exit $'changes to a subshell\'s hash table affect the parent shell'
 
+# ======
+# Variables set in functions inside of a virtual subshell should not affect the
+# outside environment. This regression test must be run from the disk.
+testvars=$tmp/testvars.sh
+cat >| "$testvars" << 'EOF'
+c=0
+function set_ac { a=1; c=1; }
+function set_abc { ( set_ac ; b=1 ) }
+set_abc
+echo "a=$a b=$b c=$c"
+EOF
+v=$($SHELL $testvars) && [[ "$v" == "a= b= c=0" ]] || err_exit 'variables set in subshells are not confined to the subshell'
+
 # ======
 exit $((Errors<125?Errors:125))