exec-invoke: Always go via stdin fd in setup_pam() to get tty

exec-invoke: Always go via stdin fd in setup_pam() to get tty

We might have resolved the tty to something else if it was set to
/dev/console, so let's always go via stdin in setup_pam(). This also
means we won't set the pam tty if only stdout or stderr are connected
to a tty, which seems like a sensible thing to do.