Specify the location of the executable for the URL rewriter.
	Since they can perform almost any function there isn't one included.

	For each requested URL rewriter will receive on line with the format

	URL <SP> client_ip "/" fqdn <SP> user <SP> method <SP> urlgroup
	 [<SP> kvpairs] <NL>

	In the future, the rewriter interface will be extended with
	key=value pairs ("kvpairs" shown above).  Rewriter programs
	should be prepared to receive and possibly ignore additional
	whitespace-separated tokens on each input line.

	And the rewriter may return a rewritten URL. The other components of
	the request line does not need to be returned (ignored if they are).

	The rewriter can also indicate that a client-side redirect should
	be performed to the new URL. This is done by prefixing the returned
	URL with "301:" (moved permanently) or 302: (moved temporarily).

	It can also return a "urlgroup" that can subsequently be matched
	in cache_peer_access and similar ACL driven rules. An urlgroup is
	returned by prefixing the returned URL with "!urlgroup!".

	By default, a URL rewriter is not used.