lockdown passwordStretching parameters

Strictly validates the `passwordStretching` parameters to only allow:

```js
{
  type: 'PBKDF2/scrypt/PBKDF2/v1',
  PBKDF2_rounds_1: 20000,
  scrypt_N: 65536,
  scrypt_r: 8,
  scrypt_p: 1,
  PBKDF2_rounds_2: 20000
}
```

This should address the immediate concerns of #400 and #401 but the flexibility of these values remains to be defined.

@rfk @warner r?