core: Adjust for TOFU_STATS change in gnupg 2.1.16.

* src/gpgme.h.in (_gpgme_tofu_info): Change 'firstseen' and 'lastseen'
to a timestamp value.
* src/verify.c (parse_tofu_stats): Do not cap these values at UINT_MAX.

Using an unsigned long here is okay: We will never get an error and
even on machines where unsigned long is 32 bit (e.g. Windows64) this
allows us to operate until 2106. By then Windows will be a footnote
in history or Windows128 has changed that type to something larger
than 32 bit ;-)

• Signed-off-by: Werner Koch <wk@gnupg.org>