sandbox: also restore terminal settings when killed by SIGINT

sandbox: also restore terminal settings when killed by SIGINT

Hitting Ctrl-C is a documented way to exit the sandbox, but it is not
actually equivalent to the reset command. The latter, since it follows
normal process exit, takes care to reset terminal settings and
restoring the O_NONBLOCK behaviour of stdin (and, in a terminal, that
is usually the same file description as stdout and stderr, i.e. some
/dev/pts/NN).

Failure to restore (remove) O_NONBLOCK from stdout/stderr can cause
very surprising and hard to debug problems back in the terminal. For
example, I had "make -j8" consistently failing without much
information about just exactly what went wrong, but sometimes I did
get a "echo: write error". I was at first afraid my disk was getting
bad, but then a simple "dmesg" _also_ failed with write error - so it
was writing to the terminal that was buggered. And both "make -j8" and
dmesg in another terminal window worked just fine.

So install a SIGINT handler so that if the chosen terminal
mode (cooked or raw-with-sigs) means Ctrl-C sends a SIGINT, we will
still call os_fd_restore(), then reraise the signal and die as usual
from SIGINT.

Before:

$ grep flags /proc/$$/fdinfo/1
flags:  0102002
$ ./u-boot
# hit Ctrl-C
$ grep flags /proc/$$/fdinfo/1
flags:  0106002

After:

$ grep flags /proc/$$/fdinfo/1
flags:  0102002
$ ./u-boot
# hit Ctrl-C
$ grep flags /proc/$$/fdinfo/1
flags:  0102002

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>