This is three-quarters NIIBE and one-quarter me, but I though it would
be easier to have a single complete patch that applies to
STABLE-BRANCH-2-0 than two patches floating around.

This is basically NIIBE Yatuka's patch here:

http://lists.gnupg.org/pipermail/gnupg-devel/2010-November/025828.html

Plus another check that closes down the reader explicitly if we get a
SW_HOST_NO_READER error, so that an unplugged reader doesn't hold onto
the first slot, preventing us from getting to the active plugged reader.