This is a wishlist item for the implementation of what was discussed
recently on emacs-devel, namely using a larger output buffer, and
avoiding fflush as a means of improving tty display update speed.

The patch below is my first take on that.  It adds a low-level interface
consisting of two functions tty--set-output-buffer-size and
tty-output-buffer-size to change and retrieve the output buffer size of
a device.  A buffer size of 0 is taken to mean "do what we did since
1991, and a non-zero value is used as the buffer size set with setvbuf.

Default is currently to do everything as it always was.  One has to do
something like

  (tty--set-output-buffer-size (* 64 1024))

to change to the new behavior.

Feedback welcome, especially for the default, i.e. leaving everything
as-is, and for the Lisp interface.