auth_pam_x11: support font selection (#15)

auth_pam_x11: support font selection

Usage:

- Find a good font name with xfontsel.
- Launch the screen saver via something like:

```
XSECURELOCK_FONT=-misc-fixed-medium-r-normal--20-200-75-75-c-100-iso10646-1 xsecurelock 
```

If the font can for some reason not be loaded, we're still falling back to fixed to allow SOME unlocking.