Add ‘termios-failure?’

Most termios(3) functions return 0 on success, -1 on failure. This
function checks for that.

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
cf-set-speed!: Stop when the first FFI call fails

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Add access to errno

Most of the functions signal errors by returning an integer < 0.
The kind of error is communicated via ‘errno’. So in order to
be able to properly react to errors, access is kind of required.

I don't mind doing it in here. But in the long run, something
like this should be provided in a central place. Be it guile-core,
or be it some "guile-posix" library.

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Add high-level module (termios with-exceptions)

This module wraps the error-producing procedures from (termios) and
throws exceptions when they happen. This forces the user to do more
error checking, hopefully.

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Test the running terminal with (termios with-exceptions)

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>