MINOR: fcgi: Move application validation to the post-check stages

MINOR: fcgi: Move application validation to the post-check stages

Filter instances will be finalized during configuration post-parsing.  FCGI
proxy validation must run after that stage to see applications supplied by
instances, rather than depending on configuration postparser ordering.

Split the existing postparser into a per-proxy post-check for the HTTP mode,
server mux and request header policy checks, and a global post-check for
application finalization. Instance lookup itself is added with the
filter-config directive in a later patch.