Merge r1800689 from trunk:

On the trunk:

mod_http2: disable and give warning when mpm_prefork is encountered. 
     The server will continue to work, but HTTP/2 will no longer be negotiated.


Submitted by: icing
Reviewed by: icing, ylavic, jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1800774 13f79535-47bb-0310-9956-ffa450edef68