Use RFC5861::stale-while-revalidate to initialize beresp.grace

Submitted by:	fgs
varnishlog -k argument

Implement -k argument in vut for use with varnishlog. This takes as an
argument the number of log transactions to be processed before
exiting. Useful to stop varnishlog after a given set of log data has
been produced.

Drop -s from the 'not implemented yet' list as this option has very
limited usefullness.
add back the graphviz sources as removed by commit 4ad0d5bdfe45eb31111fb1d0b1a4aa72942b477d
look for the dot tool from the graphviz package and ps2pdf
basic build infrastructure for graphviz output
remove size params - they are set in the Makefile now
progress on dot graphs - add cache_fetch.dot for backend side
comma instead of fullstop
better names, add error node for backend