Fix race between nosy_open() and remove_card() by replacing the
unprotected array of card pointers by a mutex-protected list of cards.

Make card instances reference-counted and let each client hold a
reference.

Notify clients about card removal via POLLHUP in poll()'s events
bitmap; also let read() fail with errno=ENODEV if the card was removed
and everything in the buffer was read.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>