Added API functions iio_buffer_first, iio_buffer_step, iio_buffer_end

Those functions can be used to process the data of a given channel
from the buffer, with the following code:

ptrdiff_t step = iio_buffer_step(buffer, channel);
void *ptr, *end = iio_buffer_end(buffer);

for (ptr = iio_buffer_first(buffer, channel);
			ptr < end; ptr += step) {
		...
}

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