Switch default version of PostgreSQL from 9.5 to 11

Two important changes arise with it:

• Default user switches from "pgsql" to "postgres"
• Default data-dir switches from "/usr/local/pgsql/data" to "/var/db/postgres/data11"

Also add a migration procedure, which takes the user-switch into account by preventing
it on database-side.

Exp-Run: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=239514

Thanks to: tobik, antoine, mfechner

Sponsored by: Professionelles Bounce Management