Merge branch 'xauth-radius-multi'

Introduces multiple rounds in the eap-radius XAuth backend, concatenating
answers to a single password to verify using a RADIUS User-Password attribute.
This is known to work fine with iOS and OS X clients, allowing two-factor
authentication with proper dialogs.

Different XAuth "profiles" for each backend can be selected using a generic
colon sperated suffix for the XAuth string.