Tracepoint API namespacing tracepoint-rcu

The ABI bump gives us the opportunity to namespace all public symbols
under the 'lttng_ust_' prefix. Namespace all API symbols and macros
under 'lttng_ust_' / 'LTTNG_UST_' and add compat macros to keep
compatibility with the previous API.

Change-Id: Ia07f15d7dd01c6d1e86033c83d329888bb52553f
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>