gbhub: make GB drops attributable, not spooky

Link-drop investigation fallout - every drop looked identical because
the hub discarded all the evidence:

- gb_reader logs *how* a GB left (clean eof vs the exception) with
  timestamps on join/leave, for correlating against SSH/terminal events.
- Spawn mode keeps each emulator's stderr in /tmp/gbhub_<n>.err instead
  of devnull - postmortems get the emulator's side of the story.
- A second gbhub exits with a clear 'already running?' message instead
  of an ioctl traceback (and can't damage the live hub's NAT/socket).

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