Unix timestamp bug in x509:setLifetime() and x509:getLifetime()

[Emilia]

Every time I'm trying to call agetLifetime() function on a certificate with expiration date less then 946684800 01/01/2000 @ 12:00am (UTC) in unix timestamp the function returns 4070908799 12/31/2098 @ 11:59pm (UTC).
The same behaviour noticed for setLifetime() when trying to set valid to date for less then 946684800.

[daurnimator]

See #179