PIN cache should be implemented in libopensc

[martin]

Currently we have two places that implement pin caching: pkcs11 and tokend. This should be brought under a common hood (brought closer to libopensc core) and implemented in 'libopensc' (though not necessarily in src/libopensc). Also, everything related to pinpads should end up in the same place (AFAIK there is no support for pinpads on osx yet, but still...)