PR23356: stap-serverd: fix private key disposal race condition

PR23356: stap-serverd: fix private key disposal race condition

Previous code made it possible for an incoming request to be processed
in a handle_connection() thread at the same time as the privkey to it
would be disposed-of in the accept_connections() loop.  (This could
happen if the latter encountered a cert-validation error immediately
after the handle_connection thread started, thus its loop was exited.)

We now pass a SECKEY_CopyPrivateKey(privKey) to the thread, which it
will dispose of itself.  Thus the main thread can do its prior thing.