Port LastlogTimes object to platforms with a 64-bit lastlog.ll_time

Fixes this error from merulo, an ia64 machine:

Traceback (most recent call last):
  File "/etc/cron.weekly/puppet-mail-big-homedirs", line 256, in <module>
    HomedirReminder().run()
  File "/etc/cron.weekly/puppet-mail-big-homedirs", line 166, in __init__
    self.lastlog_times = LastlogTimes()
  File "/etc/cron.weekly/puppet-mail-big-homedirs", line 125, in __init__
    lastlog_time, _, _ = list(struct.unpack(self.LASTLOG_STRUCT, record))
struct.error: unpack requires a string argument of length 292