Add buffered I/O back into APR's file I/O on unix. This code comes from

the OS/2 code, and has been munged to make it compile.  This has not been
tested, but unbuffered I/O still works.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@84968 13f79535-47bb-0310-9956-ffa450edef68