Delay to turn off the screen after wrong passwd

Also add flag for number of seconds to wait for it.

When user cancels the password or enters a wrong one, i3lock will wait
for some time before putting the monitors back to sleep. By default it's
30 seconds, but this flag allows to control that.
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.
Fix typo
Ignore empty passwd after repeated Enter keypress

When no ignore_empty_password flag is specified, but Enter key is held
down for some time, i3lock enters a finite, but hours-long loop
verifying empty password. Don't do that, skip empty password
verification if nothing was pressed after last Enter keypress.

Again, making the software cat-proof.
Fix typo in manual