auth_pam_x11: Add some more knobs for burn-in mitigation.

XSECURELOCK_AUTH_TIMEOUT (default: 300): number of seconds to wait for
user input on an auth prompt. This actually matches the previous
hardcoded setting, but is configurable now.

XSECURELOCK_BURNIN_MITIGATION (default: 16): do not always display the
auth prompt at the same place, but allow it to be off center by up to
the given amount of pixels. Helps when the auth screen times out but
reappears for some reason (e.g. a vibrating desk surface moving the
mouse).

XSECURELOCK_BURNIN_MITIGATION_DYNAMIC (default: off): allow the auth
prompt move during a single auth session. Many will find this annoying,
but it really should fix (by blurring out) any burn-in effects.