If overlay encryption is required between peers, the fast datapath
forwarder returns an error rather than aborting the process, allowing
the overlay switch to fall back gracefully to an encrypted sleeve
overlay connection.
This is mostly a matter of moving some files, changing package names,
changing some imports, and changing some type references.
We duplicate the odd constant & global var, and the intmac/macint
functions, since neither 'router' or 'mesh' nor, really,' common' are
a natural home for them.
...since the main connections only need to know about the former.
We make LocalConnection.Overlay (formerly '.forwarder') public since
the router needs access to it, and it shouldn't be doing that via a
private field.
Martynas Pumputis