add mod_proxy_compat.h

This captures most of the 2.4 -> 2.2 compatibility issues in
mod_proxy_fcgi.c.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
mod_proxy_fcgi: adjust interface for worker->io_buffer_size

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
mod_proxy_fcgi: adjust passed error structure

Need to have access to conf->error_override, which in 2.2 is part of the
proxy_server_conf structure, and not the proxy_dir_conf structure. Since
there's no other user to the latter in this code path, just pass the
proxy_server_conf struct.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>