master: enable specific gfx generation via cron

munin-graph usually reads the graphs to be generated from a fixed file,
$DBDIR/graphs. Now it can also be launch with all the graphs that it needs to
generate as arguments.
master: contrib tool for cron multithread gfx

In the 2.0 line, the munin-graph was single-threaded by design : if you have a
big install you should really move to CGI-based graphing.

Several users rightfully complained about their need of a multithreaded
cron-based munin-graph, since they had a quite special install.

So here is a tool that does it.
Merge branch 'cron-gfx-multi' into devel