This commit adds support to simulate Thread radio over IPv6.
With this commit, a simulation will be simulated over either IPv6 or IPv4.
If it's simulated on IPv6, it communicates with other simulation nodes
in IPv6 group `ff02::116`. And if it's simulated on IPv4, it communicates
with other simulation nodes in IPv4 group `224.0.0.116`.
Note that simulating virtual time is not included in this commit.
Yakun Xu