Before this change we initialized the rc for a single VFS. However
rclone can have multiple VFSes in use now so this is no longer
adequate.
This change adds an optional fs parameter to all the VFS methods to
disambiguate VFSes when there is more than one in use.
It also adds a method vfs/list to show all the active VFSes.
This adds outline tests for the rc commands which didn't have tests
before.
