auth_pam_x11: measure the prompt timeout in realtime, not monotonic time.

Yes, this means auth_pam_x11 may instant time out when the clock is
stepped forward by NTP. Trying this out anyway, as:

- This should only be an issue if the clock is stepped forward by more
  than few minutes, as the timeout defaults to 5 minutes.
- We also put in a specific counter measure against the clock stepping
  backwards - when it does, we simply reset the timeout.

Advantage: that way, auth_pam_x11 will almost always exit on resuming
from suspend state. This is very desirable so we can control visibility
of the hostname in a wrapper script.