s3: smbd: POSIX ACLs: support for MS NFS style uid, gid and mode

Add optional support for MS NFS style mapping. uid, gid and mode can
be queried with get_secinfo, mode can be changed with
set_secinfo. This is only enabled by smb2/create context AAPL
capability negotiation atm.

Signed-off-by: Ralph Boehme <rb@sernet.de>