cwdstuff: Don't leave from setting the CWD prematurely on init

cwdstuff: Don't leave from setting the CWD prematurely on init

There are certain, very obscure scenarios, which render the Windows
CWD handle inaccessible for reopening.  An easy one is, the handle can
be NULL if the permissions of the CWD changed under the parent processes
feet.

Originally we just set errno and returned, but in case of init at
process startup that left the "posix" member NULL and subsequent
calls to getcwd failed with EFAULT.

We now check for a NULL handle and change the reopen approach
accordingly.  If that doesn't work, try to duplicate the handle instead.
If duplicating fails, too, we set the dir handle to NULL and carry on.
This will at least set posix to some valid path and subsequent getcwd
calls won't fail.  A NULL dir handle is ok, because we already do this
for virtual paths.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>