Avoid magic numbers. Since we are reading the header, let's

be explicit about it. Also removes the need to clean up
the readbuf again, and any potential for confusion on
what we are doing ;)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/fcgi-proxy-dev@365376 13f79535-47bb-0310-9956-ffa450edef68