Make username default to current login name

[WGH-]

This is the way libpq (and other libraries based on it) does this:

• https://github.com/postgres/postgres/blob/REL_15_2/src/interfaces/libpq/fe-connect.c#L5947-L6056

At least some third-party libraries not based on libpq do that as well:

• https://github.com/jackc/pgx/blob/v5.3.1/pgconn/defaults.go#L18-L23
• https://github.com/lib/pq/blob/v1.10.8/connector.go#L103-L112

I think it would make sense to do it here for compatibility with other libraries.

[WGH-]

Maybe I'll submit a PR for this in following weeks