add multi-thread stuff for OpenSSL

Register the multi-thread required callbacks for OpenSSL as suggested in SOFTHSM-36 (BTW if this is good for correctness it doesn't fix the ticket reported bug as it is not in backends, and for Botan multi-threading is handled at the initialisation and IMHO without any flexibility, i.e., using unconditionally OS mutexes vs. SoftHSM mutexes).

(Note: I resets the fork master to eliminate change-set incoherence)