purge: fix tests to allow empty config vars
backup: Save config files if they are outside of PGDATA

If found outside $PGDATA, like in Debian, postgresql.conf, pg_hba.conf
and pg_ident.conf are backed up to a conf/ subdirectory.

They are restored to $PGDATA/restored_config_files, a message is added
to inform the user where they are at the end of the action.