Without this fix the response of nohttp sites to HTTP requests changes
depending on the existence of at least one HTTP enabled site:
* no HTTP enabled sites -> connection refused
* at least one HTTP enabled site -> 503
This fix ensures the response is always 503.
Before, a fallback http server was created to handle requests for
unknown virtual hosts even when `HTTPS_METHOD=nohttp`. (In this case,
all http vhosts would be unknown.) Likewise, a catch-all fallback
https server was still created even if `HTTPS_METHOD=nohttps`.
Now the fallback servers are created only if needed. This brings the
behavior in line with the documentation and user expectation. It will
also make it easier to implement a planned feature: different servers
on different ports.