I think this is a better i/o layer interface.

- move main/iol_unix.c to os/unix/iol_socket.c
- create main/iol_file.c ... use APR methods eventually
- remove iol_data from ap_iol, instead the ap_iol is just embedded into
  the private data of the i/o layer
- remove ap_bpushfd(), replace with ap_bpush_iol()


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@83375 13f79535-47bb-0310-9956-ffa450edef68