Merge r729586, r732504 from trunk:

CGI: return 504 (Gateway timeout) rather than 500 when a script
times out before returning status line/headers.
PR 42190


Improve canned 504 error message in the light of r729586 and covener's comment.

Submitted by: niq
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@733757 13f79535-47bb-0310-9956-ffa450edef68