Decrement login API version numbers. Proper use of rpc ValueOf.
Remove rpcreflect ValueOfVersion, checking anonRoot rootName is "Admin"
Login v0 and v1 share common implementation.

Underlying impl supports v1 login API. v0 wrapper maps the new response
type onto the older API result.

Added unit test coverage of v1 client failure mode on v0 server. Should
fail even for <1.20 server, as the response type does not match.