Actually the basic idea of the callback design was stolen from the 
GNU Privacy Guard. 


The idea is to make it easy as possible for software which don't need
any low-level functionality so it just use the high-level API. For example
if the package just want to use the cdk_file_xxx functions but it needs
a way to present some information to the end-user.


We use a simple callback function for this purpose:

        callback (void * opaque, int type, const char * data)

The @opaque value can be used for a handle that stores various data
and structs needed by the callback function itself.

The @type is the type of status. Opencdk.h defines all types which
are accessable by the end-user.

The @data contains all needed information as a string.


Defined status messages
-----------------------

CDK_CB_PUBKEY_ENC = 1       [long keyid] [keytype] [keylength]
        Used when a public key encrypted packet was parsed.
