s3: smbd: In filename_convert_dirfsp(), allow SMB1+POSIX to traverse non-terminal...

s3: smbd: In filename_convert_dirfsp(), allow SMB1+POSIX to traverse non-terminal symlinks.

This is the behavior of filename_convert() and
we need to allow it for the legacy SMB1+POSIX libsmbclient
libraries already deployed out there.

When we add SMB2 POSIX we must disallow symlink
traversal over any symlinks, the client must
resolve symlinks locally.

Add a note to show this is where we need to add
an error for SMB2+POSIX names with UCF_POSIX_PATHNAMES
set.

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>