Add support for creating a private network

This functionality is not designed to securely isolate machines on the
bitmessage network and relies on all machines agreeing to participate in
a private network. This patch allows for local addresses to be used
within the bitmessage network and not bootstrapping with the bitmessage
network.
Fix the comment for the name of the local IPv6 address range
Make the -L option work for IPv6 local addresses too

The unique local address range is supposed to work as roughly the
equivalent to the private address range in IPv4. This patch makes it
filter this IPv6 range based on the -b option too.