CVE-2020-25720: s4-acl: Change behavior of Create Children check

CVE-2020-25720: s4-acl: Change behavior of Create Children check

Up to now, the rights to modify an attribute were not checked during an LDAP
add operation. This means that even if a user has no right to modify
an attribute, they can still specify any value during object creation,
and the validated writes were not checked.
This patch changes this behavior. During an add operation,
a security descriptor is created that does not include the one provided by the
user, and is used to verify that the user has the right to modify the supplied attributes.
Exception is made for an object's mandatory attributes, and if the user has Write DACL right,
further checks are skipped.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14810

Pair-Programmed-With: Joseph Sutton <josephsutton@catalyst.net.nz>

Signed-off-by: Nadezhda Ivanova <nivanova@symas.com>
Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>