fs: Pin created backends until parents are finalized

This attempts to solve the backend lifecycle problem by

- Pinning backends mentioned on the command line into the cache
  indefinitely

- Unpinning backends when the containing structure (VFS, wrapping
  backend) is destroyed

See: https://forum.rclone.org/t/rclone-rc-backend-command-not-working-as-expected/18834

fs/cache/cache.go

@@ -2,6 +2,7 @@
 package cache
 
 import (
+	"runtime"
 	"sync"
 
 	"github.com/rclone/rclone/fs"
@@ -80,6 +81,18 @@ func Pin(f fs.Fs) {
 	c.Pin(fs.ConfigString(f))
 }
 
+// PinUntilFinalized pins f into the cache until x is garbage collected
+//
+// This calls runtime.SetFinalizer on x so it shouldn't have a
+// finalizer already.
+func PinUntilFinalized(f fs.Fs, x interface{}) {
+	Pin(f)
+	runtime.SetFinalizer(x, func(_ interface{}) {
+		Unpin(f)
+	})
+
+}
+
 // Unpin f from the cache
 func Unpin(f fs.Fs) {
 	c.Pin(fs.ConfigString(f))