When the serial console process exited on its own (a bad token, an
unreachable host, a killed consolecallback), the read task died on EIO
while fd and child_pid stayed set: the child was a zombie and every later
connection wrote to the dead pty until socketiod was restarted. A child
that failed before exec also returned into the server code and sent its
traceback to the client.
Now the read loop treats EOF/EIO as the end: it reaps the child, closes
the pty, tells the clients "[console closed]" and disconnects them, so
the next connection starts a new console. The child always leaves with
os._exit, also without a token; output is decoded incrementally, and the
unused imports are gone.
