Probe different credentials to list databases

[tbussmann]

Currently, Postgres.app does try to connect to the selected server to show its databases. This is done on the configured port using a unix domain socket connection with the username of the logged in user to a database named accordingly (default connection credentials).

We could extend this to try different common combinations like postgres / postgres or $USER / postgres and with a TCP connection in case the default way fails.

[jakob]

I've added a fallback to use postgres / postgres in 0eaceed.

This should cover the use case when creating the $USER database fails, or when $USER is deleted

[jakob]

A potential drawback of the fallback is that databases might show up in Postgres.app, but double clicking them would lead to an error message.