PostgreSQL: only show databases you can access

[thomas-daniels]

When connecting to a PostgreSQL database, Adminer's dropdown also includes all databases you don't have access to, which is not very useful. This commit filters out databases that you don't have the USAGE privilege on, to present a cleaner overview of available databases.

[Baltix]

Original project is no longer maintained, please send your commits to AdminerEvo - https://github.com/adminerevo/adminerevo