mon: client: multifs: enable clients to map a filesystem name to a FSCID
Currently clients have to specify the ID of the filesystem they want to connect to (or specify no ID to get the legacy filesystem).
It would be much friendlier to let them specify a name. Add a way for clients to get a mapping of name to ID from the mons. The mapping should respect any auth caps limiting the client's access to filesystems, and not tell the client about any filesystems that it doesn't have access to.