s3:auth: explicitly add BUILTIN\Guests to the guest token

s3:auth: explicitly add BUILTIN\Guests to the guest token

This changes ensures that smbd always adds BUILTIN\Guests to the guest token
which is required for guest authentication.

Currently the guest token depends on the on-disk configured group mappings. If
there's an existing group mapping for BUILTIN\Guests, but LOCALSAM\Guest is not
a member, the final guest token won't contain BUILTIN\Guests.

For SMB2 the flag SMB2_SESSION_FLAG_IS_GUEST will not be set in the final SMB2
SESSION_SETUP response, because smbd sets it based on the token containing the
BUILTIN\Guests SID S-1-5-32-546.

At the same time, the packet is not signed which causes Windows clients and
smbclient to reject the unsigned SMB2 SESSION_SETUP response.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13944

Pair-programmed-with: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Jun  5 16:55:26 UTC 2019 on sn-devel-184