Get PostgreSQL engine working with Target.

Note that the deprecated username, password, host, port, and db_name keys in
the core.pg config have been removed, since Target does not support them.

Related changes:

* `registry_destination` is now a true alias for `destination`, rather than
  `uri->name`.
* Un-deprecated the `--db-*` options. I think they are just too useful and
  ingrained. I might change my mind again, though.
* Fixed Target where it was checking the wrong variable for the presence of
  a name.