http: Enforce consistently no response body with both 204 and 304 statuses.

Provide AP_STATUS_IS_HEADER_ONLY() helper/macro to check for 204 or 304 and
use it where some special treatment is needed when no body is expected.

Some of those places handled 204 only.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1837056 13f79535-47bb-0310-9956-ffa450edef68