net/openldap26-server: update to 2.6.13.
While I'm there also address the following issues:
- Add nssov overlay (PR 293531)
- Add an option to delay slapd startup (PR 280849)
- Support PREFIX/openldap/slapd.d/cn=config/olcDatabase=* as directory (PR 261792 / 258418)
PR: 258418, 261792, 280849, 293531