Add a new API call to check whether the filesystem is attached to card10 or to a connected USB host (in which case it is unavailable to card10). Make the menu check this and automatically exit "USB storage mode" when a host ejects the device.
Automatically exit the "USB storage mode" once the connected USB host ejects card10.
Add an `os.fs_is_attached()` function to allow apps to check whether the filesystem is available.
