Supporting fixed length keypad input

[NIIBE Yutaka]

On 2013-01-15 at 11:36 +0900, NIIBE Yutaka wrote:
> To summarize discussion, I'd like to propose the following for pinpad
> input.
> 
>   * Default is variable length pinpad input when reader supports the
>     feature.
> 
>   * Use pinentry by keyboard on host PC, when reader doesn't supports
>     the feature (including reader supports pinpad input but requires
>     fixed length input).
> 
>   * Only when a user wants to do special thing, he needs to specify
>     this.  Special cases are:
> 
>     (1) Use pinentry by keyboard even with pinpad reader.
>         (for cases when PIN has characters other than digits.)
> 
>     (2) Use fixed length input.

Now, in scd-work branch, it is implemented in this way.

I'd like to merge scd-work branch to master.  It will be backported
to 2.0 branch, too.

After that, I will do function name changes for keypad -> pinpad.
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