Two friends with keys to the Temple could not see each other.  The phone
reached the Temple and the laptop reached the Temple, and between them
lay the wall.  WgForward; takes it down, if you want it down.

- A packet out of the tunnel for an address in another friend's allowed
  IPs is sealed again for him, ip_forward the way Linux does it.  TTL
  goes down by one, the header checksum is done again, a packet out of
  hops dies here.  Never back to the one who sent it.
- Only between friends.  Nothing goes out to the internet, no NAT.
- Off by default: a friend's key opens the Temple, not every other
  friend.  WgSave; keeps it, WgRep; counts it.
- Each friend must route the whole tunnel to the Temple, AllowedIPs
  10.66.66.0/24 on his side.  The README says so.

wg_interop.py G: a second Linux joins.  Without WgForward the two do not
meet, with it they ping, ttl=63, and talk TCP through the Temple.  It
puts back any ~/Wg.HC it finds, and the test begins with WgDown;
because a Temple with ~/Wg.HC came up with its own tunnel and the first
WgUp said already up.  That cost me an evening.  21 of 21.