tools: iio: iio_generic_buffer: allow continuous looping

Sometimes it's useful to stream samples forever, such as when
stress-testing a driver overnight to check for memory leaks or other
issues. When the program receives a signal, it will gracefully cleanup,
so it is still safe to terminate at any time.

Add support for specifying a negative -c option, meaning that we should
loop forever. To do so, we need to use a long long (instead of just
long) for num_loops so that current code specifying num_loops greater
than UNSIGNED_LONG_MAX doesn't break.

Signed-off-by: Martin Kelly <mkelly@xevo.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>