Make i3lock support PAM prompts (better support for PAM messages and passwordless PAM methods like fingerprint-gui)

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Other (Please describe in detail)

## Current Behavior
Currently you need to submit a password to trigger any PAM method, so just fingerprint won't work. For some reason I actually need to supply both a valid password and fingerprint when fingerprint-gui is enabled.

## Expected Behavior
Unlock with just fingerprint, or just password.

## Environment
Output of `i3lock --version`:

i3lock version:  2.10 (2017-11-25)