Fixed ngx_gmtime() on 32-bit platforms with 64-bit time_t. In ngx_gmtime(), instead of casting to ngx_uint_t we now work with time_t directly. This allows using dates after 2038 on 32-bit platforms which use 64-bit time_t, notably NetBSD and OpenBSD. As the code is not able to work with negative time_t values, argument is now set to 0 for negative values. As a positive side effect, this results in Epoch being used for such values instead of a date in distant future.