Support old-style shell for rebar3 shell

This is quite the hack.

This requires to detect the current shell running; if it's the new
shell, business as usual.

However, if it's the old shell, we have to find a way to take over it
and drive IO. This requires a few steps because:

- the old shell does not let you be supervised intelligently (it uses
  supervisor_bridge, so killing the child is not a supported operation
  from the supervisor)
- the old shell ignores all trappable exit signals except those coming
  from the Port in charge of stdio ({fd, 0, 1})
- the old shell shuts down on all exit signals from the stdio Port
  except for badsig, and replicates the shutdown reason otherwise
- An escript does not tolerate the `user` process dying (old shell) for
  any non-normal reason without also taking the whole escript down
- Booting in an escript has an implicit 'noshell' argument interpreted
  by the old shell as a way to boot the stdio Port with only stdout
  taken care of

Because of all these points, we have to kill the old `user` process by
sending it a message pretending to be the Stdio port dying of reason
`normal`, which lets it die without triggering the ire of its
supervision tree and keeping the escript alive. This, in turn, kills the
old stdio port since its parent (user.erl) has died.

Then we have to boot our copy of user.erl (rebar_user.erl) which
conveniently ignores the possibility of running the stdio port on stdout
only -- always using stdin *and* stdout, giving us a bona fide old-style
shell.

A known issue introduced is that running r3:do(ct) seems to then kill
the shell, and r3:do(dialyzer) appears to have an odd failure, but
otherwise most other commands appear to work fine.

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