Discard half-entered passwd after some inactivity

Discard half-entered passwd after some inactivity

When you start typing a password and then wander off, let's discard that
attempt after a few minutes of inactivity (and turn off the monitors if
dpms is on).

This is a real actual case of making the software cat-proof: when my cat
treads over the keyboard at night, I don't want the monitors to shine
for the rest of the night.