Tracepoint API namespacing '__lttng_ust_events_init'

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: I2918a0974bd0c7d0c44a4d383d2092d586a20892
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>