http_request.[ch]:

*) add the "install_filter" hook as a hook/control point for modules to
   install their filters. [Ryan Bloom]

http_protocol.c:
*) move check_first_conn_error() up in the file; no actual changes
*) add checked_bputstrs(), checked_bflush(), and checked_bputs(). These are
   copies of ap_rvputs(), ap_rflush(), and ap_rputs() respectively. The
   users of the checked_* functions will be independent of filtering changes
   to the ap_r* functions.
*) add flush_filters() place holder


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85685 13f79535-47bb-0310-9956-ffa450edef68