When the database_or_uri is not a URI, rather than generate a dsn string
just collect the various psycopg2.connect keyword arguments in a
dictionary, and return that. This way psycopg2/libpq is the one doing
any necessary processing on the various parameters.

Fixes #3865 and fixes #6958 in a more resilient manner than manually
escaping dsn-injected values so database names or passwords containing
spaces or non-ascii characters work correctly.

In case a URI is provided to the dsn() function, return it as the `dsn`
keyword to get the same behaviour as previously.

Since we're checking dsns by equality, a dict should work just as well
as a string.