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/tests/variables.sh

@@ -1011,6 +1011,20 @@ $SHELL -c '
 e=$?
 ((e == 1)) || err_exit "Failure in making one or more special variables readonly in a subshell (exit status $e)"
 
+# ... subshell leak test
+$SHELL -c '
+	errors=0
+	for var
+	do	if	eval "($var=bug); [[ \${$var} == bug ]]" 2>/dev/null
+		then	echo "	$0: special variable $var leaks out of subshell" >&2
+			let errors++
+		fi
+	done
+	exit $((errors + 1))
+' subshell_leak_test "$@"
+e=$?
+((e == 1)) || err_exit "One or more special variables leak out of a subshell (exit status $e)"
+
 # ======
 # ${.sh.pid} should be the forked subshell's PID
 (