pam: Handle sigint by using a signalfd

It makes sense to allow interrupting fingerprint authentication, but PAM does not provide a way to define an interruptable operation.

We can work around this somewhat though by at least reacting to SIGINT in an interactive terminal. Obviously, we shouldn't override the signal handler, because that would be too intrusive. But creating a signalfd is easy enough and doesn't affect the rest of the process state as much.