net/389-ds-base: Fix instance creation

Add a dedicated dirsrv user and group.

Use FreeBSD-compatible paths for runtime files, databases, backups,
LDIF files, and logs. Create the persistent database and log parent
directories with suitable permissions.

Detect getpeereid(3) during configuration and fix peer credential
handling for LDAPI SASL EXTERNAL authentication.

Sponsored by: Netzkommune GmbH