really fix Rasmus' chunking problem this time

introduce LARGE_WRITE_THRESHOLD tunable for determining when large_write()
is a good idea

Reviewed by:	Rasmus Lerdorf


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@80031 13f79535-47bb-0310-9956-ffa450edef68