databases/openldap*: move from net/

Move all OpenLDAP ports (openldap25-client, openldap25-server,
openldap26-client, openldap26-server) from net/ to databases/ to be
consistent with OpenLDAP 2.7 which was already in databases/.

Bump PORTREVISION for all moved ports. Simplify Mk/Uses/ldap.mk now
that all OpenLDAP versions live under databases/.