Add os:isatty/1 as a bif

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.

This PR adds os:isatty/1 as well as docs and a test of it's argument
handling. os:isatty/1 accepts the atoms stdin, stdout and stderr as well
as any 32 bit non_negative_integer() representing a file descriptor. It
simply returns false in case the integer is not an FD, and throws in the
case of a bad argument. The test does not check the return value in the
valid case, because that would depend upon how tests are run.