Add PAM message capability

[nununoisy]

I initially wondered why i3lock would freeze for a few seconds before accepting my password. I had a hunch that it was fprintd waiting for my finger, and sure enough I tried it and it allowed me in.

To avoid this, I decided it would be helpful to add the ability to see messages from pam_conv. The feature is optional and disabled by default. It can be enabled by passing the argument -m.

In the future it might be helpful to use a libev timer to ensure messages are readable but for now this works very well.