Framework: add an add_graph function, and support a scalar autoconf value.

The former is useful when graphs are conditionals and you don't know
how many you're adding.

The latter allows to just set the vaue instad of having to provide a
function, when autoconf comes "for free" with discovery.
Merge load and uptime plugins into procfs.

The new plugin with the new Framework code makes it cleaner to just
have one place where most of the /proc files are read.
procsys: implement a new plugin to replace entropy, open_files and open_inodes.

This new plugin uses the new Framework to create graphs for all the
sysctl-related data as a signle round-trip.

There is only one change compared to the previous plugins: open_inodes
produces a more meaningful graph of allocated and free inodes, rather
than the previous one that used a fake max value (as it was actually
the kernel's allocation).

Both open_files and open_inodes graphs now use areastacks instead of lines.
procfs: improve docs.
plugins: rewrite new multigraph plugins for using the Framework