Make dlopen more robust by using the .0 target for the library

We've experienced a double library load on a setup where the lttng-ust
.so and .so.0 were two physical files instead of a symlink (as they
should be). So let's make liblttng-ust slightly more robust by
specifying the target .so.0 instead of .so. It will also make handling
of library version upgrades easier.

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