lib/gettime.c: gettime(): Call a2i() instead of strtoull_noneg()

lib/gettime.c: gettime(): Call a2i() instead of strtoull_noneg()

time_t isn't necessarily unsigned (in fact, it's likely to be signed.
Therefore, parse the number as the right type, via a2i(time_t, ...).

Still, reject negative numbers, just to be cautious.  It was done
before (strtoull_noneg()), so it shouldn't be a problem.  (However,
strtoull_noneg() was only introduced recently, and before that we called
strtoull(3), which silently accepted negative values.)

Remove the limitation of ULONG_MAX, which seems arbitrary.  It probably
was written in times where 'time_t' had the same length of 'long', and
this was thus a test that the value didn't overflow 'time_t'.  Such a
test is implicit in the a2i() call, so forget about it.

Unify the error messages into a single one that provides all the info
(except the value of 'fallback').

Link: <https://github.com/shadow-maint/shadow/commit/cb610d54b47ea2fc3da5a1b7c5a71274ada91371#r136407772>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: Chris Lamb <lamby@debian.org>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>