Fix minor typeset attribute regressions (re: 95fe07d, fdb9781)

This fixes the following regressions marked TODO in attributes.sh:

$ typeset -L 13 bar; readonly bar; typeset -p bar
typeset -r -L 0 foo		# exp.: typeset -r -L 13 foo
$ typeset -R 13 bar; readonly bar; typeset -p bar
typeset -r -R 0 bar		# exp.: typeset -r -R 13 bar
$ typeset -Z 13 baz; readonly baz; typeset -p baz
typeset -r -Z 0 -R 0 baz	# exp.: typeset -r Z 13 -R 13 baz

I've discovered that these were briefly fixed between fdb9781 (Red
Hat patch for typeset -xu/-xl) and 95fe07d (reversal of patch,
different -xu/-xl fix, but reintroduced these regressions).

src/cmd/ksh93/sh/name.c: nv_newattr():
- Replace check from 95fe07d with a new one that combines its
  approach with that of fdb9781: do not change size (and hence
  return early) if NV_RDONLY and/or NV_EXPORT are the only
  attributes that are changing.

src/cmd/ksh93/tests/attributes.sh:
- Enable the TODO regression tests.

src/cmd/ksh93/sh/name.c

@@ -2966,11 +2966,10 @@ void nv_newattr (register Namval_t *np, unsigned newatts, int size)
 			nv_onattr(np,NV_EXPORT);
 			sh_envput(shp->env,np);
 		}
-		if((n^newatts)==NV_EXPORT && !trans)
-			/* Only EXPORT attribute has changed and thus all work has been done. */
-			return;
 	}
 	oldsize = nv_size(np);
+	if(((n^newatts) & ~(NV_EXPORT|NV_RDONLY))==0)
+		size = oldsize;
 	if((size==oldsize|| (n&NV_INTEGER)) && !trans && ((n^newatts)&~NV_NOCHANGE)==0)
 	{
 		if(size>0)

src/cmd/ksh93/tests/attributes.sh

@@ -526,16 +526,16 @@ typeset -A expect=(
 	[F12]='typeset -x -r -F 12 foo'
 	[H]='typeset -x -r -H foo'
 	[L]='typeset -x -r -L 0 foo'
-#	[L17]='typeset -x -r -L 17 foo'		# TODO: outputs '-L 0'
+	[L17]='typeset -x -r -L 17 foo'
 	[Mtolower]='typeset -x -r -l foo'
 	[Mtoupper]='typeset -x -r -u foo'
 	[R]='typeset -x -r -R 0 foo'
-#	[R17]='typeset -x -r -R 17 foo'		# TODO: outputs '-L 0'
+	[R17]='typeset -x -r -R 17 foo'
 	[X17]='typeset -x -r -X 17 foo'
 	[S]='typeset -x -r foo'
 	[T]='typeset -x -r foo'
 	[Z]='typeset -x -r -Z 0 -R 0 foo'
-#	[Z13]='typeset -x -r -Z 13 -R 13 foo'	# TODO: outputs 'typeset -x -r -Z 0 -R 0 foo'
+	[Z13]='typeset -x -r -Z 13 -R 13 foo'
 )
 for flag in "${!expect[@]}"
 do	unset foo