Fixed pkcs#1 unpadding

Depending on the size of the used key the old code was not able to
cope with the missing leading zero byte of a pkcs#1 frame. This is
due to the fact that we use MPIs for conveying the data and our MPIs
usually strip leading zero bytes. The changed code should now behave
identical to the code used by GnuPG.

Also added a few more comments.