ENT-4353 Rewrite PostgreSQL migration

Split some code into functions

Rewrite migration from 6 levels of nested if's into function with return's

Make $BACKUP_DIR configurable and $PREFIX/state/pg/backup by default

Do not fail when $BACKUP_DIR is on a separate partition

Treat all pg_*.conf files with same respect as pg_hba.conf

Check for errors

Author: Aleksei Shpakovskii