loadparm: check for AD DC required VFS modules

loadparm: check for AD DC required VFS modules

When Samba is running as a domain controller and the "vfs objects"
parameter is not set, then the dfs_samba4 and acl_xattr modules are
automatically enabled.
However, if the "vfs objects" is defined, then the setting is left
as-is. This means that attempts to us other VFS modules have the side
effect of disabling the dfs_samba4 and acl_xattr modules, causing
unexpected behaviour, which is then blamed on the VFS modules that were
explicitly defined.

This change ensures that when running as a domain controller, Samba logs
an error if the required VFS modules are not enabled by an explicit
"vfs objects" definition.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10560

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Björn Jacke <bjacke@samba.org>