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.