scd: Simplify inclusion of app-common.h.

* scd/scdaemon.h: Include app-common.h.  Remove inclusion of that
header from all other files.
(card_t, app_t): Move typedef to ...
* scd/app-common.h: here.  Use them in the defs.

In another patch we will need apptype_t in the ctrl object and thus we
need to reorganize things a bit now. Given that most files need
app-common anyway it makes sense to always include it.

• Signed-off-by: Werner Koch <wk@gnupg.org>