nfscommon: Add some support for POSIX draft ACLs

An internet draft (expected to become an RFC someday)
https://datatracker.ietf.org/doc/draft-ietf-nfsv4-posix-acls
describes an extension to NFSv4.2 to handle POSIX draft ACLs.

This is the third of several patches that implement the
above draft.

There should be no semantics change for the series at
this point.

Fixes: a35bbd5d9f5f ("nfscommon: Add some support for POSIX draft ACLs")