Tracepoint API namespacing 'tracepoint_dlopen'

Tracepoint API namespacing 'tracepoint_dlopen'

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: I3349b267ee0c1e119fed509f6e600bc8edb27f1c
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>