[erlang-patches] Add os:isatty/1

[Andrew Stone]

Hi All,

I've added support for the posix 'isatty' function to the os module. This
is a relatively minor addition, however it makes me wonder if there is more
interest from the OTP team in adding other useful posix functions. I know
personally, I'd love to have access direct from erlang for more posix
functions. Let me know what you think.

*https://github.com/erlang/otp/pull/480
<https://github.com/erlang/otp/pull/480>*

Thanks,
Andrew
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# Why do we need this feature?
The posix libc function, isatty(), is a useful function that will tell
you if a file descriptor is attached to a terminal. If for instance you
want to write a program that uses ansi color codes, it is not enough to
just check $TERM because the output could be redirected to a file. In
that case you would end up with the escape codes in the file making it
harder to read and parse. By calling os:isatty/1 with stdout as it's
parameter the program can ensure that it never writes escape codes to
log files.

# Risks or Uncertain Artifacts
It uses a posix function that should be available on all platforms including
Windows. The call is deprecated in favor of the C++ standard _isatty() call on
Windows though. 

# How did you solve it?
I added a bif to the os module. This seems to be the only reasonable way to do
it, as the call must be in the same os process of the caller.