ⓘ Follow up on #2794 by making `redirect_port` work even when `redirect_host` is not set. Keep existing host precedence and correctly handle proxy headers, IPv6 addresses, default ports, and invalid port values.
ⓘ Fix linked-server by keeping the child URL recognizable to the parent, then generating the final WebSocket URL on the same host and port the browser is using. This keeps the connection working with the current Webmin session.
ⓘ Use the existing scheduled websocket cleanup path for linked-server ws-link routes and expire unopened temporary routes after five minutes.
This limits how long credential-bearing proxy routes can remain in `miniserv.conf` while leaving active and normal websocket cleanup behavior unchanged.
ⓘ Require websocket routes to opt in with allow_basic_ws before Basic auth is accepted in session mode. Mark linked ws-link routes and no-cookie backend-session routes as allowed, while leaving normal session-backed routes unmarked.
ⓘ Correct linked-server WebSocket proxy registration for parent-prefixed URLs, rebuild backend Host/Origin from the child server, and prevent duplicate rewrites from invalidating tokens.
This PR adds general WebSocket proxying for linked Webmin servers, allowing modules such as `xterm` to work when opened through `servers/link.cgi`.
As requested in https://github.com/webmin/webmin/issues/1866.
