Soft tracing with lttng-ust.

Insertion of tracepoints at the legacy SCOPE_ON/OFF points.
This is a first implementation.
Thread : fosi_internal clean free of names and zeroing after freeing.
Thread : add setName method.

The implementation is currently available on gnulinux only.
implement trace for all Hook methods in TaskContext
tracing: add the task name to the trace events
tracing: add tracing to ports
Merge branch 'lttng' into lttng-2.7