Let a client resume its session with an OpenSSL mTLS server

Without a session id context, OpenSSL answers a client resuming its
session with a fatal alert once client certificates are verified, which
curl and browsers run into on their second connection. Set one on every
server context. A changed client CA still stops resumption, as fcf7568
flushes the cache and rotates the ticket keys.

httplib.h

@@ -19812,6 +19812,10 @@ inline ctx_t create_server_context() {
                                  SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION |
                                  SSL_OP_NO_RENEGOTIATION);
     SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION);
+    // Without one, a client resuming its session is sent a fatal alert once
+    // client certificates are verified.
+    static const unsigned char sid_ctx[] = "cpp-httplib";
+    SSL_CTX_set_session_id_context(ctx, sid_ctx, sizeof(sid_ctx) - 1);
   }
   return static_cast<ctx_t>(ctx);
 }