s3/smbd: ensure global "smb encrypt = off" is effective for share with "smb encrypt...

s3/smbd: ensure global "smb encrypt = off" is effective for share with "smb encrypt = desired"

If encryption is disabled globally, per definition we shouldn't allow
enabling encryption on individual shares.

The behaviour of specifying

[Global]
  smb encrypt = off

[share]
  smb encrypt = desired

must be an unecrypted tree connect to the share "share".

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12520

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>