The multicast example Demo/sockets/mcast.py
1. mentions that multicast is only implemented on SGI (and optional on
other systems). That is not the case anymore.
2. Includes completely unrelated code for broadcast transmission. There
is already an example for that, suitably named broadcast.py (in the same
directory).
3. Does only support IPv4.
4. Is borderline buggy. Line 85 only works by accident. Similarly, line
37 fails if the TTL would be increased to >127.
5. Is ugly. Line 79-81 are an example of how not to write python code,
line 62 can be deleted without changing the program's semantics.
(All line numbers refer to rev70006)

The attached patch removes broadcast support in mcast.py, adds IPv6
support and fixes all other problems mentioned above.