fix: Handle the errors when the listener socket is closed gracefully.

[hessjcg]

The accept loop for socket connections now will properly handle the error case where the listener socket is closed gracefully.

Fixes #2372

[enocom]

Nice find! This probably deserves a test, if only of the private method, so that future maintainers don't trip over this again.

[hessjcg]

I'm converting this back to a draft, because after some testing, the errors returned from l.Accept() on socket close are not exactly as documented.