Let chdir be overriden with FCGI_CHDIR
Let chdir be overriden with FCGI_CHDIR

chdir to directory which holds scripts doesn't work well with some CGIs.
Let the FastCGI invoker provide `FCGI_CHDIR` which can be `-` to inhibit
chdir, or another place to chdir to.

gnosek/fcgiwrap,muthhus/fcgiwrap