MINOR: haload: add FastCGI support

Add -fcgi (protocol selector) and --fcgi-app <str> (raw passthrough
into a generated "fcgi-app haload" section, mainly for "docroot").

The FCGI filter is attached from hld_cfg_finalize() (post-check),
not hld_pre_check(): servers don't exist yet at pre-check time, and
HAProxy's own postparser then wrongly reports no FCGI server found.

"set-param"/"pass-header" have no effect (no struct stream to run
them); use -H ":fcgi-<param>:<value>" instead.