Honor httpd_suppress_version_string in more contexts (#1121)

Honor httpd_suppress_version_string in more contexts (#1121)

* HTML "signature" part of generated HTTP responses to URN requests
* Server header in generated HTTP redirect responses
* boundary strings in generated HTTP 206 multipart/byteranges responses

Also document known contexts outside the configuration directive reach.
We may assert that all of them are outside the current "httpd" scope
(implied by the directive name), but admins may not know how we define
that "httpd" scope; being more explicit may reduce surprises.