Fix passport-local

Correctly handle form POST credentials. Also, return the passport instance from init() in case the app needs this later.

Here's a basic provider config:
```
{
  "local-login": {
    "provider": "local",
    "module": "passport-local",
    "authPath": "/auth/local",
    "successRedirect": "/",
    "failureRedirect": "/",
    "session": true
  }
}
```