[core] config opt to intercept dynamic handler err (fixes #974)

[core] config opt to intercept dynamic handler err (fixes #974)

new directive server.error-intercept = [ "enable" | "disable" ]
to intercept 4xx and 5xx responses from dynamic handlers
(e.g. CGI, FastCGI, SCGI, proxy)

Intercepted HTTP error status are then handled by one of
  server.error-handler
  server.error-handler-404
  server.errorfile-prefix
(if configured)

Do not use server.error-intercept with locations handled by mod_webdav!

x-ref:
  "would like something similar to nginx proxy_intercept_errors"
  https://redmine.lighttpd.net/issues/974