Improve timestamp support in BSD archive files to avoid linker

Improve timestamp support in BSD archive files to avoid linker
warnings.

* libbfd.h (struct artdata):  Add armap_timestamp and armap_datepos.
(bfd_flush, bfd_stat):  Add prototypes.
* libbfd.c (bfd_flush):  Add, does fflush on a BFD.
(bfd_stat):  Add, does fstat on a BFD.

* archive.c (_bfd_write_archive_contents): At end of file writing,
verify and possibly update the timestamp in the armap, if a BSD
archive.  FIXME!  Kludge recognizes BSD archive, rather than
vectoring properly.  Should add to xvec.
(compute_and_write_armap):  Move prototype to top, avoid PROTO.
(bsd_write_armap): Save timestamp and file location thereof, when
writing the armap in a BSD file.
(bsd_update_armap_timestamp):  New function to check and
rewrite the timestamp.