fix: PSK broker integration did not work

This commit resolves multiple issues with the PSK broker integration.

- The manual testing procedure never actually utilized the brokers
  due to the use of the outfile option, this led to issues with the
  broker being hidden.
- The manual testing procedure omitted checking whether a PSK was
  actually sent to WireGuard entirely. This was fixed by writing an
  entirely new manual integration testing shell-script that can serve
  as a blueprint for future integration tests.
- Many parts of the PSK broker code did not report (log) errors
  accurately; added error logging
- BrokerServer set message.payload.return_code to the msg_type value,
  this led to crashes
- The PSK broker commands all omitted to set the memfd policy, this led
  to immediate crashes once secrets where actually allocated
- The MioBrokerClient IO state machine was broken and the design was
  too obtuse to debug. The state machine returned the length prefix as
  a message instead of actually interpreting it as a state machine.
  Seems the code was integrated but never actually tested. This was
  fixed by rewriting the entire state machine code using the new
  LengthPrefixEncoder/Decoder facilities. A write-buffer that was not
  being flushed is now handled by flushing the buffer in blocking-io
  mode.

rosenpass/Cargo.toml

@@ -68,7 +68,7 @@ procspawn = {workspace = true}
 tempfile = { workspace = true }
 
 [features]
-experiment_memfd_secret = []
+experiment_memfd_secret = ["rosenpass-wireguard-broker/experiment_memfd_secret"]
 experiment_broker_api = ["rosenpass-wireguard-broker/experimental_broker_api", "command-fds"]
 experiment_libcrux = ["rosenpass-ciphers/experiment_libcrux"]
 experiment_api = ["hex-literal"]