mod_cgid: Continuation of r1862968, experimental fd passing support.

Split out CGI bucket implementation from mod_cgi and use in both
mod_cgi and mod_cgid, bringing stderr handling in mod_cgid up to par
with mod_cgi.  (There is a lot of code which has been copied between
mod_cgi{,d} so there's scope for further reduction of source
duplication between the modules using this header)

* modules/generators/cgi_common.h: Copied from mod_cgi.c, removed
  everything but the CGI bucket implementation with only one change:
  (struct cgi_bucket_data, cgi_bucket_create, cgi_bucket_read): Take a
  timeout on bucket creation, store and use on reads.

* modules/generators/mod_cgi.c [APR_FILES_AS_SOCKETS]: Include
  cgi_common.h.
  (cgi_handler): Pass configured timeout to CGI bucket.

* modules/generators/mod_cgid.c: Include cgi_common.h.
  (log_script_err): Copy from mod_cgi.c.
  (log_script): Use log_script_err.
  (send_req): Take fd for stderr.
  (cgid_child_errfn): Handle fd-passing case by writing error
  to stderr for client to pass through ap_log_rerror.
  (cgid_handler): Create pipe for stderr, pass write-end to
  server via send_req, use read-end to create CGI bucket.  Handle
  stderr output in failure paths.

PR: 54221


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1863191 13f79535-47bb-0310-9956-ffa450edef68
