Add experimental support for fd passing in mod_cgid. Attaches CGI

Add experimental support for fd passing in mod_cgid.  Attaches CGI
script stderr to the error log specific to the vhost, by passing the
appropriate fd over the AF_UNIX socket from the request handling
thread to the cgid server process.

* modules/generators/config5.m4: Add --enable-cgid-fdpassing.

* modules/generators/mod_cgid.c (sock_readhdr): New function, also
  returns auxiliary control data (the stderr fd) if available.
  (sock_write): Take optional aux fd argument, send it as control
  data.  (send_req, get_req): Adjust accordingly to pass/receive the
  stderr fd.
  (cgid_server): Use passed fd if available, limit the lifetime.

PR: 60692

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1862968 13f79535-47bb-0310-9956-ffa450edef68