Make the piped_log structure opaque:

* include/http_log.h: Remove piped_log structure definition here;
  (ap_piped_log_read_fd, ap_piped_log_write_fd): Turn macros into
  functions.             

* server/log.c: Move piped_log structure definition here; replace
  fds[2] with read_fd, write_fd.
  (ap_piped_log_read_fd, ap_piped_log_write_fd): New functions.
  (throughout...): Replace use of ap_piped_log_write_fd, 
  ap_piped_log_read_fd, with direct reference to structure fields.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@775120 13f79535-47bb-0310-9956-ffa450edef68