radius-admin: use += operator in radgroupcheck
radius-admin: use ':=' operator for Group

Use := operator for Group attribute in radgroupcheck.

It's a "dummy"/unnecessary row in the table, useful for an
attribute-less / stub group.

Using '=' will result in a forced conversion to '==',
which means a match we certainly don't want...

(no access-request RADIUS packet from the NAS contains a Group
attribute!)
version bump 2013.09.02