Improve TTY handling

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improve TTY handling based on MORE Fix (Issue #27 )

• Tasks get a PROCESS GROUP (PGRP), inherited from parent
• Fork can create new/use PGRP so tasks can be grouped into jobs by shell (creted PGRP Id is same as task ID of forked child, so returned task ID value can be reused)
• STREAMS get an optional "line discipline"
  -- if set
  --- STREAM gets a PGRP attached
  ---- On GETC, if PGRP does not match block
  ---- On PUTC,
  ----- if Ctrl-C -> signal (all) PGRP tasks SIGTERM
  ----- if Ctrl-D -> EOF

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New FORK param: to-be-used PGRP (or flag to create new one)

We cannot re-use Task ID of created child as PGRP ID when a new one is created - a previously started PGRP may still run, but this very PGRP member has already finished, and Task ID would be reused. Maybe second return param?

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This ticket should include making slipd the controlling terminal for users logged in via telnet

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Started working on design in https://github.com/fachat/GeckOS-V2/blob/master/GeckOS-NG-Terminal.adoc

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Streams can now have a foreground process attached that receives the signal sent to the stream. The console sends SIG_BRK on Ctrl-C, and the lib6502 signal handler terminates the process by default. This can be overwritten by setting a lib6502 signal handler. The lsh currently does that (and ignores it).

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Blocking when reading a stream by another task than the foreground task (if set) is not there (yet). Not sure when / if this will be needed, so deferred for now