Only get non-parameter parts of @ARGV to @graphs (Fix #1418)

* munin-graph expects ether no cli parameter at all or a specific string which
  graph to render. When called with --debug or any other parameter it throws
  error messages to the log and does not render the graphs.

  This patch changes this. It pushes only params from @ARGV to @graphs which
  doesn't start with --.
  See trac ticket #1418 .