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