Investigate multithreaded access to PKCS#11

[martin]

Allow multiple threads to access pkcs#11 module (and as well libopensc?)

[stef]

The problem is currently that one thread could do a C_Finalize() which causes the sc_context_t struct to be released. But if another thread is stilling waiting in C_WaitForSlotEvent(), it will call/work with an invalid sc_context_t (-> crash, ...)

This is the case in e.g. Firefox 1.5 which switched from polling for slot events in the main thread, to opening a new thread and doing a C_WaitForSlotEvent() in that thread.

[nils]

The real problem is that its not even clear which opensc function should be reentrant (and which objects could be shared among different threads). Before this hasn't been specified (and of course this should be well documented) is doesn't even make sense to think about using opensc in a multi-threaded environment like a pkcs11 lib.