buffer .var maps to avoid asking the kernel for one byte at a

time

note that a system trace of .var map processing still shows an
extra read() after we hit EOF the first time; we could make
use of the EOF flag in APR to avoid the read() or play with
mod_negotiation

Suggested by:	Marc Slemko


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@90609 13f79535-47bb-0310-9956-ffa450edef68