as they want.

Append to already existing header if possible.

NB:   If you return early from your own vcl_recv, without pasting these
lines in top of your vcl_recv, your backend gets no X-F-F header.

Fixes	#601
Fixes	#540



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4467 d4fa192b-c00b-0410-8231-f00ffab90ce4