Link client side log records to backend side log records with the "Backend"
record.

Content is:
	Fd# of backend connection
	VCL name of director
	VCL name of chosen backend.

Spotted by:	sky



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@3212 d4fa192b-c00b-0410-8231-f00ffab90ce4