scd: pinpad input improvements

[NIIBE Yutaka]

For the issue 1549, I commit and pushed my changes for scd/apdu.c.
The bug was introduced by me when I added PINMIN/PINMAX handling
and I forgot to remove old code (which should have removed).

   https://bugs.g10code.com/gnupg/issue1549

Also, I created a wiki page at:

   http://wiki.gnupg.org/CardReader/PinpadInput

If people have other card readers, please add information.
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