Ensure that slapd starts before kdc, as the kdc may be configured to
require LDAP services. If it is configured to require LDAP and the
slapd server is not yet started, the kdc will fail to start.

PR: 229939
Approved by: delphij@ (maintainer)
MFH: 2018Q3
Differential Revision: https://reviews.freebsd.org/D16602