Introduce API compatibility macros

Introduce API compatibility macros, allowing to compile-out backward
compatibility for older API major versions.

The initial use-case introduced here is renaming of the tracepoint(),
do_tracepoint() and tracepoint_enabled() macros to add the lttng_ust_
prefix. The old macros are only defined when API compatibility with
soname 0 is enabled.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I39e5eb4ae6ceea91dd039d52c429a791bb11ce66