Stop extra buffering when stdout isn't a tty

Right now if you're running a program with its output piped to some location and
the program decides to go awry, when you kill the program via some signal none
of the program's last 4K of output will get printed to the screen. In theory the
solution to this would be to register a signal handler as part of the runtime
which then flushes the output stream.

I believe that the current behavior is far enough from what's expected that we
shouldn't be providing this sort of "super buffering" by default when stdout
isn't attached to a tty.
auto merge of #10270 : alexcrichton/rust/no-super-buffer, r=brson

Right now if you're running a program with its output piped to some location and
the program decides to go awry, when you kill the program via some signal none
of the program's last 4K of output will get printed to the screen. In theory the
solution to this would be to register a signal handler as part of the runtime
which then flushes the output stream.

I believe that the current behavior is far enough from what's expected that we
shouldn't be providing this sort of "super buffering" by default when stdout
isn't attached to a tty.