mountlib: rc: fix mounts created with mountPoint "*" overwriting each other

On Windows, passing "*" as mountPoint to the mount/mount RC command
auto-assigns a drive letter (e.g. "Z:"), but the resolved letter was
never propagated back to mountlib. This caused liveMounts to be keyed
on the literal "*", breaking tracking of multiple mounts and making
unmount unreliable.

Change MountFn to return the actual mount point as an additional
return value. Update MountPoint.Mount() to store the resolved value,
and mountRc() to use it as the liveMounts key. The mount/mount RC
response now returns the actual mountPoint so callers can discover
which drive letter was assigned.

vfs/vfstest_test.go

@@ -19,12 +19,12 @@ func TestFunctional(t *testing.T) {
 	if *fstest.RemoteName != "" {
 		t.Skip("Skip on non local")
 	}
-	vfstest.RunTests(t, true, vfscommon.CacheModeOff, true, func(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (unmountResult <-chan error, unmount func() error, err error) {
+	vfstest.RunTests(t, true, vfscommon.CacheModeOff, true, func(VFS *vfs.VFS, mountpoint string, opt *mountlib.Options) (unmountResult <-chan error, unmount func() error, actualMountpoint string, err error) {
 		unmountResultChan := make(chan (error), 1)
 		unmount = func() error {
 			unmountResultChan <- nil
 			return nil
 		}
-		return unmountResultChan, unmount, nil
+		return unmountResultChan, unmount, mountpoint, nil
 	})
 }