* tub.port is now an Endpoint server specification string (which is pretty
  much just like a strports string, but can be extended by plugins). It now
  rejects "tcp:0" and "0". The tahoe.cfg value overrides anything stored on
  disk (in client.port). This should have no effect on most nodes, which do
  not set tub.port in tahoe.cfg, and wrote an allocated port number to
  client.port the first time they launched. Folks who want to listen on a
  specific port number typically set tub.port to "tcp:12345" or "12345", not
  "0".
* The "portnumfile" (e.g. NODEDIR/client.port) is written as soon as the port
  is allocated, before the tub is created, and only if "tub.port" was empty.
  The old code wrote to it unconditionally, and after Tub startup. So if the
  user allows NODEDIR/client.port to be written, then later modifies
  tahoe.cfg to set "tub.port" to a different value, this difference will
  persist (and the node will honor tahoe.cfg "tub.port" exclusively).
* We now encourage static allocation of tub.port, and pre-configuration of
  the node's externally-reachable IP address or hostname (by setting
  tub.location). Automatic IP-address detection is deprecated. Automatic port
  allocation is merely discouraged. Eventually both will be managed by "tahoe
  create-node", but for now we recommend users edit their tahoe.cfg after
  node creation and before first launch.
* "tahoe start" now creates the Tub, and all primary software components,
  before the child process daemonizes. Many configuration errors which would
  previously have been reported in a logfile (after node startup), will now
  be signalled immediately, via stderr. In these cases, the "tahoe start"
  process will exit with a non-zero return code.