Add --passstderr flag to forward STDERR through WebSocket

[Formatted]

Summary

Adds a --passstderr CLI flag that forwards STDERR output from the wrapped process to WebSocket clients as tagged JSON messages.

Addresses: #403 (Feature Request: STDERR redirected through websockets, open since 2021)

Problem

Currently, STDERR output from subprocesses is silently swallowed — it's only logged on the server side and never reaches the WebSocket client. This makes debugging scripts nearly impossible, as warn/die in Perl, Python tracebacks, and other error output is invisible to clients.

Solution

When --passstderr is enabled:

• STDERR lines are forwarded as JSON: {"stream":"stderr","data":"error message"}
• STDOUT lines are also tagged for consistency: {"stream":"stdout","data":"output line"}
• The output channel closes only after both STDOUT and STDERR complete, ensuring no messages are lost

When --passstderr is not set (default), behavior is unchanged — STDERR is only logged server-side.

Example

websocketd --port=8080 --passstderr ./myscript.sh

Client receives:

{"stream":"stdout","data":"normal output"}
{"stream":"stderr","data":"warning: something went wrong"}

Client-side usage

ws.onmessage = function(event) {
  var msg = JSON.parse(event.data);
  if (msg.stream === 'stderr') {
    console.error(msg.data);
  } else {
    console.log(msg.data);
  }
};

Changes

• libwebsocketd/config.go — Added PassStderr field to Config
• libwebsocketd/process_endpoint.go — Added passStderr parameter to NewProcessEndpoint, new readStdoutTagged/readStderrTagged methods with sync.WaitGroup, and tagMessage helper with proper JSON escaping
• libwebsocketd/handler.go — Wire PassStderr config through to NewProcessEndpoint
• config.go — Added --passstderr CLI flag
• help.go — Updated help text
• README.md — Mentioned new flag in features list
• CHANGES — Added changelog entry
• libwebsocketd/process_endpoint_stderr_test.go — 3 unit tests covering the feature

Testing

• All 217 existing tests pass
• 3 new tests added:
  1. STDERR forwarded as JSON when passStderr enabled
  2. STDERR not forwarded when passStderr disabled
  3. Output channel closes after both STDOUT and STDERR complete
• Built and vetted on Go 1.21