login: Use unsigned 32-bit types for seconds-since-epoch

These fields store timestamps when the system was running.  No Linux
systems existed before 1970, so these values are unused.  Switching
to unsigned types allows continued use of the existing struct layouts
beyond the year 2038.

The intent is to give distributions more time to switch to improved
interfaces that also avoid locking/data corruption issues.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>