Use PQconnectdbParams instead of PQsetdbLogin

[Cito]

Currently, connections in the classic and DB API 2 modules are made with PQsetdbLogin which only allows a limited number of parameters. Additional parameters must be sent as connection string or connection URI, but they cannot be passed as parameters (the DB API 2 module fakes that by creating a connection string from keyword arguments).

We may want to switch to the newer PQconnectdbParams to properly send any number of parameters.

When doing so, we should take care that everything is properly documented, backward compatible, and particularly the default values are set properly and the same as before.

(copied from Trac ticket 79, created 2019-04-24)