MINOR: fcgi-app: Use the filter configuration helpers for lookups and checks

MINOR: fcgi-app: Use the filter configuration helpers for lookups and checks

Look up the FCGI filter configuration of a proxy with flt_find_conf()
instead of scanning the legacy filter_configs and then falling back to the
finalized instances. The helper already selects the right source
according to the proxy mode.

Rework fcgi_flt_check() to walk the filter configurations with
flt_foreach_conf() and check the uniqueness of the FCGI application and
the implicit declaration rules in a dedicated callback.

The callback now considers any second FCGI filter of the proxy as an
error, including one referring to the same application, since a single
FCGI filter remains supported per proxy.