*/postgresql: Add older versions, allow parallel installation

The company I work for requires support for older versions of postgresql, as long as the version is supported upstream. Furthermore, keeping older versions also makes it easier for users to upgrade from one version to another, with the possibility to fallback to an older version if required.

This MR adds an abstraction to the executables (like initdb, postgres, psql etc) to /usr/bin, with some symlinks similar to my openjdk aports. The new "psotgresql-common" created and maintains the symlinks, keeping "/usr/lib/postgresql/default-postgres" symlinked to the most recent version.

The postgresql aports are no longer installed in a single system-wide directory (like /usr/lib/postgresql/), this directy is now versioned (/usr/lib/postgresql/pg-12 for example), allowing parallel installations.

The openrc scripts are now versioned too (postgresql -> postgresql12, pg-restore -> pg12-restore).