Hi guys,

I ran into a problem where if vlock was running over ssh and the ssh
session died my system logs would get spammed by the now orphaned vlock
process on Arch Linux:

    Sep 25 20:51:12 hostname vlock[31336]: pam_unix(vlock:auth): auth could not identify password for [user]
    Sep 25 20:51:13 hostname vlock[31336]: pam_unix(vlock:auth): auth could not identify password for [user]
    Sep 25 20:51:14 hostname vlock[31336]: pam_unix(vlock:auth): auth could not identify password for [user]

I was able to re-create this in many other situations (ssh, xterm
window/tab, tmux window/pane, etc).

The patch know checks for a tty on stdin where PAM will likely ask for
the password and exits if it's not found.