some tweaks in both the splithttp and httpupgrade packages

[us254]

I've made some tweaks in both the splithttp and httpupgrade packages. Here's what's up:

In /splithttp/hub.go:

1. Switched up the session reaper to use time.AfterFunc. Way cleaner.
2. Fixed that lock issue in httpResponseBodyWriter.Write. No more deadlocks!
3. Made ListenSH actually tell us when something's wrong.
4. Added some defer cleanup in ListenSH. No more resource leaks.
5. Threw in a dummy server for UDS. Keeps things stable.
6. Properly handled ErrServerClosed. Smooth shutdowns, you know?

And in /httpupgrade/hub.go:

1. Fixed that typo - innnerListener is now innerListener. Two n's, not three!
2. Added a quick buffer clear after reading the HTTP request. Helps with WebSocket stuff later.
3. Put the connection handling in its own goroutine. Should keep things moving if addConn gets slow.

[mmmray]

I think this entire PR, but certainly the description is AI-generated. One can argue that some of these changes are useful (typo fixes, indeed there is something wrong with uds, see #3577), but the PR description even references some changes that are definitely not present in the PR. Most changes here are missing justification.