This fixes an issue when a variable is resolved differently in two
bindings.

For instance, take the following env file:

```
PORT=8000
URL=http://localhost:${PORT}
```

With `PORT` set to `1234` in the environment, the environment resulting
from `dotenv_load(override=False)` would be:

```
PORT=1234
URL=http://localhost:8000
```

This was inconsistent and is fixed by this commit.  The environment
would now be:

```
PORT=1234
URL=http://localhost:1234
```

with override, and

```
PORT=8000
URL=http://localhost:8000
```

without override.

The behavior of `load_dotenv` is unchanged and always assumes
`override=True`.