Move common (and near-identical) code for CGI response output handling

Move common (and near-identical) code for CGI response output handling
to cgi_common.h; the diff between the modules for this code was as
follows:

https://people.apache.org/~jorton/mod_cgi-to-cgid-handler.diff

Change from previous: mod_cgi will now explicitly discard output when
returning HTTP_MOVED_TEMPORARILY for relative redirects (should not be
functionally different), TRACE1 logging of ap_pass_brigade failures
for mod_cgid is dropped.

* modules/generators/cgi_common.h (cgi_handle_response): New function,
  factored out from mod_cgid.
  (discard_script_output): Copied function from mod_cgi/d unchanged.

* modules/generator/mod_cgid.c (cgid_handler),
  modules/generator/mod_cgi.c (cgi_handler): Use cgi_handle_response.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1867968 13f79535-47bb-0310-9956-ffa450edef68