Clean up non-udpfromto code

So that it's simpler.  We also ALWAYS use udpfromto, as we assume
the library is used for IPv4 and IPv6 interfaces, not anything else.
Fix udpfromto bug on Mac OSX.

This is a NICE Mac OSX bug.  Create an interface with
two IP address, and then configure one listener for
each IP address.  Send thousands of packets to one
address, and some will show up on the OTHER socket.

This hack works ONLY if the clients are global.  If
each listener has the same client IP, but with
different secrets, then it will fail the rad_recv()
check above, and there's nothing you can do.

Linux does the right thing.