certtool endless loop in expiration_days

[Stefan Bühler]

Hi,

expiration_days = 36500

in a certtool template config leads to an endless loop as it overflows
and tries again forever.

a) it should fail with an error instead of looping; also signed integer
overflow is undefined: check it manually before instead of checking
the result. (days * 24 * 60 * 60 is where the overflow happens, as days
is only an integer)

b) it should really support large expiration values; ideally it should
also support the "no well-defined expiration date" 99991231235959Z
somehow.

regards,
Stefan