Allow LDAP modifications for entry creation

Heimdal will refuse to create new entries when an entry already exists even
if said entry has no kerberos info and is a new entry.

This patch fixes this issue by allowing object modifications even if the
flags disallow them when we are inserting a new principal on the database.

Signed-off-by: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>