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
plugins/diskstats: enhance the field ordering

adhawkins reported on IRC: "Across multiple servers with the same partition
setup, the partitions are appearing in different orders"

This commit sorts according to the name that appears in the graph instead of
the dm-XX number.