plugins: Rename plugins_linux.go to plugins_unix.go and use build tags

This allows pkg/authorization to build on OS X.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

pkg/plugins/plugins_linux.go → pkg/plugins/plugins_unix.go

@@ -1,3 +1,5 @@
+// +build !windows
+
 package plugins
 
 // BasePath returns the path to which all paths returned by the plugin are relative to.