Change default PLUGINUSER to match documentation

The most recent documentation for plugin configuration specifies that
the default plugin user is `munin`. Source releases do not currently
reflect this. Modify the provided Makefile.config so that source
releases do reflect this change by default.

Makefile.config

@@ -125,7 +125,7 @@ USER       := munin
 GROUP      := munin
 
 # Default user to run the plugins as
-PLUGINUSER := nobody
+PLUGINUSER := munin
 
 # Default user to run the cgi as
 CGIUSER := nobody