separate allowedips from routing for multipath

[Reid Rankin]

WireGuard doesn't care what address V has, because peers are defined
by their public key. If V switches from A1 to A2, upon receiving a
handshake packet from V's key from the address A2, B will start
sending packets for V out to A2. Incoming packets can arrive from any
IP; if they're from a connection with V, V's AllowedIP rules will be
applied.