Fix Argon2 options checks

The minimum for memory cost is 8 KiB per thread. Threads must be checked and set first to allow checking against the correct memory cost mimimum.
Options are now applied the following way:
- If config.php contains the setting with an integer higher or equal to the minimum, it is applied.
- If config.php contains the setting with an integer lower than the minimum, the minimum is applied.
- If config.php does not contain the setting or with no integer value, the PHP default is applied.

Signed-off-by: MichaIng <micha@dietpi.com>
Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>