mllib: use Unix.isatty

Make use of Unix.isatty instead of our TTY.isatty_stdout, as the
supported OCaml provides the former already.
mllib: remove the TTY module

The only function provided is isatty_stdout, which is no more used now
in favour of Unix.isatty.
mllib: check the right out channel for tty

Make sure to check whether the given output channel is a tty before
sending ANSI terminal codes to it, instead of always check what stdout
is.