lib: add plugin support

This enables loading plugins from RCLONE_PLUGIN_PATH if set.

lib/plugin/package.go

@@ -0,0 +1,16 @@
+// Package plugin implements loading out-of-tree storage backends
+// using https://golang.org/pkg/plugin/ on Linux and macOS.
+//
+// If the $RCLONE_PLUGIN_PATH is present, any Go plugins in that dir
+// named like librcloneplugin_NAME.so will be loaded.
+//
+// To create a plugin, write the backend package like it was in-tree
+// but set the package name to "main". Then, build the plugin with
+//
+//     go build -buildmode=plugin -o librcloneplugin_NAME.so
+//
+// where NAME equals the plugin's fs.RegInfo.Name.
+package plugin
+
+// Build for plugin for unsupported platforms to stop go complaining
+// about "no buildable Go source files".