freeradius: eap-pwd: DoS issues due to multithreaded BN_CTX access

In FreeRADIUS 3.0.x before 3.0.20, the EAP-pwd module used a global OpenSSL BNCTX instance to handle all handshakes. This mean multiple threads use the same BNCTX instance concurrently, resulting in crashes when concurrent EAP-pwd handshakes are initiated. This can be abused by an adversary as a...