Support `quiet` Flag in `pam.d` Config

[DabeDotCom]

Example: /etc/pam.d/ssh

auth   sufficient   pam_duo.so quiet

Summary of the change

A trivial update that allows admins to silence pam_info() messages

Test Plan

I added two simple tests:

1. Make sure the pam.d parser accepts the quiet flag
2. Make sure passing the quiet flag actually does silence the output «grin»

PS: I also added the flag to the pam_duo(8) manpage.