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/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.