Add RPC-L from the rxgk-afs draft

XXX Uses codepoints not allocated to us!
XXX The definition of struct PrAuthName is included inline but it should
    be provided by an implementation of pts-extended-names.
The server-side routines are stubs that just return success.

Change-Id: I84bee1235476fa224d07a65e6e1528c8e48cca80