MINOR: fcgi-app: Stop to explicitly loop on the proxy's filter_configs list

MINOR: fcgi-app: Stop to explicitly loop on the proxy's filter_configs list

The "fcgi-app" filter keyword parser and the implicit "use-fcgi-app"
declaration resolution both scanned the filter_configs list of the proxy
by hand to find a previously declared FCGI filter configuration, matching
on the filter id and the application name.

The parsers now use flt_find_conf() with a match callback, the supported
way to find a filter configuration, which walks the right list depending
on the filter mode of the proxy. An explicit "fcgi-app" declaration now
rejects the replacement of another explicit declaration instead of any
one found, the implicit configuration it replaces is removed with
flt_conf_delete(), and the one-fcgi-app-per-backend check of
"use-fcgi-app" resolves the FCGI instance directly with
flt_find_instance() instead of a manual loop on the configurations.