Add proxy plugin, that makes it possible for a single Munin plugin
to connect to a separate munin-node (or other daemon speaking the
munin-node protocol) and fetch values from there.
This is primarily useful together with multigraph (which is why
multigraph is required for it) to be able to merge multiple sets
of node data into the same stream, for example when fetching
OS statistics directly with munin-plugins and appserver statistics
through an embedded munin-node in the application server.
