OpenLDAP since version 2.5.4¹ supports sd_notify, so we should make use
of it.
Also updates the unit description and documentation with the values
upstream provides.
Starts slapd only after reaching `network-online.target`, which ensures
binding to specific ip addresses is possible, since `network.target`
only guarantees interfaces exist, but not that addressing is finished.
[1] https://bugs.openldap.org/show_bug.cgi?id=8707
885d4e047b