Update send_the_file to fall back to a read/write loop on platforms

without sendfile.  This also makes the apr_hdtr_t type available on
all platforms, regardless of whether apr_sendfile is defined or not.
Submitted by:	Brian Havard and Ryan Bloom


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86437 13f79535-47bb-0310-9956-ffa450edef68