Change well_known_admin_sid to well_known_admins_sid throughout.

    Change well_known_admin_sid to well_known_admins_sid throughout.
        * sec_acl.cc (setacl): Never set DELETE permission. Set
        FILE_DELETE_CHILD only on readable and executable directories.
        * sec_helper.cc: Add constructor for `well_known_null_sid'.
        * security.cc (get_nt_attribute): Set S_ISVTX for directories if
        FILE_WRITE_DATA and FILE_EXECUTE but not FILE_DELETE_CHILD is set.
        Add evaluation of S_ISVTX, S_ISGID and S_ISUID from NULL ACE.
        (alloc_sd): Never set DELETE permission. Set FILE_DELETE_CHILD
        only on readable and executable directories.
        Add creation of NULL ACE for S_ISVTX, S_ISGID and S_ISUID permissions.
        * security.h: Add extern declaration for `well_known_null_sid'.