SCGI patch for mathopd-1.6b9

Hi,

I have implemented an SCGI patch for mathopd-1.6b9.

You use it with:
Specials { SCGI { .scgi }}
where the .scgi file is a unix domain socket to the SCGI application.
Also, the SCGI application should have a large enough listen queue to
accomodate the number of simultaneous connections configured in mathopd,
or there is a possibility to get 503 code errors under load.

The idea is this:
* process_cgi was renamed into process_cgi_scgi. It builds the CGI
  environment as before, then calls either process_cgi or process_scgi.
* process_cgi forkexecs the CGI script. The master process keeps
  a socketpair to talk with it.
* process_scgi connects to an existing SCGI unix socket socket, and the
  master process keeps that socket to talk with it. The client_input pool
  is prefilled with the CGI environment variables converted to SCGI format.
* calling writetochild directly at the end of process_scgi is not necessary
  for correctness, but I found out that it made a large difference (35%) in
  performance when the SCGI serves small (3KB) cached replies.
* I modified get_path_info to consider cases where the open call returns
  ENXIO (as happens with unix sockets). When this happens, the finfo field
  of the request is not filled in. A new field 'valid_finfo' was added to
  track the validity of the finfo field.

P.S. the patch also backports easily to 1.5 based versions.

Hope this helps,