RADIUS authentication

The attached patch implements RADIUS authentication (RFC2865-compatible).

The main usecase for me in this is the ability to use (token based)
one-time-password systems easily with PostgreSQL. These systems almost
always support RADIUS, and the implementation is fairly simple. RADIUS
can of course be used in many other scenarios as well (for example, it
can be used to implement "only this group"-access with at least Active
Directory, something our current LDAP doesn't support. We might
eventually want to support that in our LDAP, but it's not there now)

-- 
 Magnus Hagander
 Me: http://www.hagander.net/
 Work: http://www.redpill-linpro.com/