The built-in Webserver's `on_path`, `on_query_string` and `on_url`
callbacks may be called multiple times from the parser; we must not
simply replace the old values, but need to concatenate the new values
instead.
This appears to be tricky for `on_path` due to the path normalization,
so we fail if the function is called again.
The built-in Webserver logs errors during request parsing to stderr,
but this is ignored by the php_cli_server framework, and apparently the
Webserver does not send a resonse at all in such cases (instead of an
4xx). Thus we can only check that a request with an overly long path
fails.
Closes GH-7207.
