Set unbuffered I/O also for C standard library

In input_available() we use function select(), so
we have to set as unbuffered also C library I/O
functions otherwise we can miss some input.

For instance in case GUI sends "go infinite\nstop\n" we
parse the "go infinite" but then input_available() under Linux
is unable to detect that we still have "stop" to be processed.

This is because "select" uses file descriptors instead of file
pointers. So it cannot know about the buffer associated to a file
pointer.

This patch, by BB+, should fix the problem.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>