iio-monitor: Added license information

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIOD: Catch signals to exit cleanly

Now when SIGHUP, SIGINT, SIGSEGV or SIGTERM are received, the main
socket is closed.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>