Cygwin: chmod: don't drop default ACEs from directory ACLs

Cygwin: chmod: don't drop default ACEs from directory ACLs

commit bc444e5aa4ca introduced a call to get_posix_access()
with a NULL pointer for the mode_t parameter because the value
is not needed later on... entirely ignoring the fact that the
mode_t bits are checked for the object being a directory.

In turn, the get_posix_access() call never checked for default
ACEs and returned only the standard ACEs.  Thus, every chmod call
on a directory dropped the default ACEs from its permissions, as
well as the default NULL deny-ACE used to store specific bits.
It got also impossible to set the sgid bit on directories.

Fixes: bc444e5aa4ca ("Reapply POSIX ACL changes.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>