Add a simulation test network via --simnet param.

This commit, along with recent commits to btcnet and btcwire, expose a new
network that is intended to provide a private network useful for
simulation testing.  To that end, it has the special property that it has
no DNS seeds and will actively ignore all addr and getaddr messages.  It
will also not try to connect to any nodes other than those specified via
--connect.  This allows the network to remain private to the specific
nodes involved in the testing and not simply become another public
testnet.

The network difficulty is also set extremely low like the regression test
network so blocks can be created extremely quickly without requiring a lot
of hashing power.