Use authmap instead of username for matching certificate with user

I made a patch that allows using authmap instead of username for matching a certificate with the user. This way a user can have multiple certificates and the user name can be freely chosen – for example the Finnish citizen certificate doesn't really have fields that I'd like to use in my own username.

The attached patch implements this as a new option in the settings page. The patch worked for me perfectly. In this version the user logs in with a password, goes to /login and ”logs in”, after which the certificate used for the login is attached to the user. After that the user can log in with the certificate.

Currently there is no way to show through the interface, which certificates the user is attached to or remove them – this can be done directly to the database though. Such an interface isn't too problematic to create if the feature is accepted to the module.