Merge pull request #384 from vincentbernat/feature/allow-inexistent-files

graph: allow RRD files to be missing

diff --cc doc/rrdgraph.pod
Simple merge

diff --cc src/rrd_fetch.c
Simple merge

diff --cc src/rrd_graph.c
Simple merge

diff --cc src/rrd_graph.h
Simple merge

diff --cc src/rrd_tool.c
Simple merge