Add quiet option to silence "deny" messages

[DabeDotCom]

Summary of the change

Nothing big; it's pretty much just like it says on the tin — allow, e.g., /etc/pam.d/sshd to use:

auth   sufficient   pam_duo.so quiet
                               ^^^^^ 

...and thereby not display the failure message back to the user.

Test Plan

I added TestPamQuiet and TestPamdConfQuiet unit tests. (Honestly, the tests account for more of the PR than the actual code change itself!)

PS: And FWIW, I also documented it in the pam_duo(8) man page.