autoreconf --install --force: replace install-sh with no timestamp

autoreconf --install --force: replace install-sh with no timestamp

Old versions of install-sh did not have a timestamp line.  Therefore,
treat the absence of a timestamp line as indicating a very old file
(that --install --force should replace), not as an error.

Problem reported by Pascal Terjan.

* bin/autoreconf.in (extract_time_stamp): Return 1970-01-01 when
  no timestamp line is found.
* tests/torture.at (Missing auxiliary files (--force)):
  Test replacement of old install-sh with no timestamp line.