It's still not possible to pick a network from the command-line, so this
is a bit broken, but the daemon component should handle it correctly at
least.
In a world where the daemon can manage more than one network, the Daemon
is really responsible only for knowing which networks are currently
joined, creating/joining/leaving networks, and routing incoming RPC
requests to the correct network handler as needed.
The new network package, with its Network interface, inherits most of
the logic that Daemon used to have, leaving Daemon only the parts needed
for the functionality just described. There's a lot of cleanup done here
in order to really nail down the separation of concerns between the two,
especially around directory creation.
