Add detach_process and detach_multi_process for backward compatibility

Because some 3rd party plugins use these methods in their code:

```ruby
class HTTPServer # w/o inheriting Fluent::Input, Fluent::Output
  include DetachMultiProcess

  def start
    detach_multi_process do
      # Do something to run HTTP server
    end
  end
end
```

lib/fluent/compat/detach_process_mixin.rb

@@ -17,9 +17,17 @@
 module Fluent
   module Compat
     module DetachProcessMixin
+      def detach_process
+        log.warn "#{__method__} is not supported in this version. ignored."
+        yield
+      end
     end
 
     module DetachMultiProcessMixin
+      def detach_multi_process
+        log.warn "#{__method__} is not supported in this version. ignored."
+        yield
+      end
     end
   end
 end