* "ex_data" - a CRYPTO_EX_DATA structure in the ENGINE structure itself
   that allows an ENGINE to store its own information there rather than in
   global variables. It follows the declarations and implementations used
   in RSA code, for better or worse. However there's a problem when storing
   state with ENGINEs because, unlike related structure types in OpenSSL,
   there is no ENGINE-vs-ENGINE_METHOD separation. Because of what ENGINE
   is, it has method pointers as its structure elements ...  which leads
   to;

 * ENGINE_FLAGS_BY_ID_COPY - if an ENGINE should not be used just as a
   reference to an "implementation" (eg. to get to a hardware device), but
   should also be able to maintain state, then this flag can be set by the
   ENGINE implementation. The result is that any call to ENGINE_by_id()
   will not result in the existing ENGINE being returned (with its
   structural reference count incremented) but instead a new copy of the
   ENGINE will be returned that can maintain its own state independantly of
   any other copies returned in the past or future. Eg. key-generation
   might involve a series of ENGINE-specific control commands to set
   algorithms, sizes, module-keys, ids, ACLs, etc. A final command could
   generate the key. An ENGINE doing this would *have* to declare
   ENGINE_FLAGS_BY_ID_COPY so that the state of that process can be
   maintained "per-handle" and unaffected by other code having a reference
   to the same ENGINE structure.