Rename data_available to data_pending

This is just a rename and a change of semantic.

The lttng_data_pending returns 0 if _NO_ data is pending meaning that
the buffers are ready to be read safely. A value of 1 means that data is
still pending so the buffers are not ready for any read.

This is the same semantic as lttng_data_available but in reverse order.

Signed-off-by: David Goulet <dgoulet@efficios.com>