tstest/natlab/vnet: add multi-NIC node support, DHCP fixes, and VIPs

Multi-NIC support:
  - Add nodeNIC type and node.extraNICs for secondary network interfaces
  - Add netForMAC/macForNet to route packets to the correct network by MAC
  - Update initFromConfig to allocate a MAC + LAN IP per network
  - Fix handleEthernetFrameFromVM, ServeUnixConn to use netForMAC
  - Fix MACOfIP, writeEth, WriteUDPPacketNoNAT, gVisor write path, and
    createARPResponse to use macForNet (return the MAC actually on that
    network, not the node's primary MAC)
  - Fix createDHCPResponse for multi-NIC (correct client IP and subnet)
  - Add nodeNICMac for secondary NIC MAC generation
  - Add Node accessors: NumNICs, NICMac, Networks, LanIP

DHCP fixes:
  - Include LeaseTime, SubnetMask, Router, DNS in DHCP Offer (not just
    Ack). systemd-networkd requires these to accept an Offer.
  - Fix DHCP response source IP: use gateway IP instead of echoing
    the request's destination (which was 255.255.255.255 for discovers)

New VIPs:
  - cloud-init.tailscale: serves per-node cloud-init meta-data, user-data,
    and network-config for VMs booting with nocloud datasource
  - files.tailscale: serves binary files (tta, tailscale, tailscaled)
    registered via RegisterFile for cloud VM provisioning
  - Add ControlServer() accessor for test control server

This is necessary for a three-VM natlab subnet router
integration test, coming later.

Updates #13038

Change-Id: I59f9f356bae9b5509c117265237983972dfdd5af
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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