Make file I/O and network I/O writev/sendv APIs consistent.

Eliminate use of ap_iovec_t and use Posix struct iovec. I never
did hear a case (much less a strong case) supporting the need
for ap_iovec_t, so out it goes.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84657 13f79535-47bb-0310-9956-ffa450edef68