tablespaces correctly, and is quite restricted on objects covered (only
tables and databases, but not tablespaces and indexes).

The attached patch contributes:

- database_size(name)
- relation_size(text)
These are the well-known functions, tablespace-aware.

- pg_tablespace_size(oid)
- pg_database_size(oid)
- pg_relation_size(oid)
Tablespace-aware implementations, used by the upper functions.
pg_relation_size will report sizes of indexes as well.

- pg_size_pretty(bigint)
Formatting of sizes, to display '146MB' instead of '152885668'

Andreas Pflug