cd: Fork if $PWD exists but is not actually the PWD (re: d1483150)

Commit d1483150 did not fully fix #153.
Test case from Harald van Dijk that was still failing:

$ mkdir test
$ cd test
$ rmdir $PWD
$ mkdir $PWD
$ ksh -c "(cd /); pwd"
/

Forking a virtual subshell in that case is needed to avoid ending
up in a directory that replaced the PWD, because it will not be
possible for a process to change back to the original directory.

src/cmd/ksh93/bltins/cd_pwd.c:
- When deciding whether to fork, instead of attempting to opendir
  the PWD, compare the inodes $PWD and "." to determine if $PWD
  still actually refers to the current directory. This uses the
  test_inode() function which is also used by 'test foo -ef bar'.

src/cmd/ksh93/tests/subshell.sh:
- Add test based on the above.

Progresses: https://github.com/ksh93/ksh/issues/153

src/cmd/ksh93/bltins/cd_pwd.c

@@ -37,7 +37,6 @@
 #include	"name.h"
 #include	"builtins.h"
 #include	<ls.h>
-#include	<ast_dir.h>
 
 /*
  * Invalidate path name bindings to relative paths
@@ -99,10 +98,7 @@ int	b_cd(int argc, char *argv[],Shbltin_t *context)
 	if(shp->subshell && !shp->subshare)
 	{
 #if _lib_fchdir
-		DIR *testdir;
+		if(!test_inode(nv_getval(pwdnod),e_dot))
-		if(testdir = opendir(nv_getval(pwdnod)))
-			closedir(testdir);
-		else
 #endif
 			sh_subfork();
 	}

src/cmd/ksh93/tests/subshell.sh

@@ -904,6 +904,15 @@ exp="PWD=$PWD"
 got=$( { "$SHELL" -c '(cd /; (cd /)); print -r -- "PWD=$PWD"'; } 2>&1 )
 ((!(e = $?))) && [[ $got == "$exp" ]] || err_exit 'failed to restore nonexistent PWD on exiting a virtual subshell' \
 	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+cd "$tmp"
+mkdir recreated
+cd recreated
+tmp=$tmp "$SHELL" -c 'cd /; rmdir "$tmp/recreated"; mkdir "$tmp/recreated"'
+exp="PWD=$PWD"
+got=$( { "$SHELL" -c '(cd /); print -r -- "PWD=$PWD"'; } 2>&1 )
+((!(e = $?))) && [[ $got == "$exp" ]] || err_exit 'failed to restore re-created PWD on exiting a virtual subshell' \
+	"(got status $e$( ((e>128)) && print -n / && kill -l "$e"), $(printf %q "$got"))"
+cd "$tmp"
 
 # ======
 exit $((Errors<125?Errors:125))