Run filter "init" functions exactly once per request. No longer run

Run filter "init" functions exactly once per request.  No longer run
init functions for connection filters (doing an "init" once per
handler invocation makes no sense for a connection filter).  No longer
run init functions multiple times per request if a subrequest is used.

* include/util_filter.h (ap_filter_rec_t): Clarify use of the init
  function pointer.

* server/config.c (invoke_filter_init): Drop ap_ prefix for private
  function; take a request_rec pointer and only invoke filters with
  matching request.
  (ap_invoke_handler): Adjust accordingly.

PR: 49328
Reviewed by: rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@953311 13f79535-47bb-0310-9956-ffa450edef68