databases/openldap27-{client,server}: add OpenLDAP 2.7.0
Add new ports for OpenLDAP 2.7.0 (Feature Release) under databases/
category, which better reflects LDAP's nature as a directory database.
Notable changes from 2.6:
- Perl and SQL backends have been removed upstream
- New resultstats contrib overlay for per-operation result code statistics
- slapi-plugin.h header no longer shipped with client library
Update Mk/Uses/ldap.mk to support version 27 with databases/ category
routing (27+ -> databases/, older -> net/), and add 27 as a possible
value in Mk/bsd.default-versions.mk.