Rename liblttng-ust-malloc to liblttng-ust-libc, update to lttng-ust 2.0

Rename liblttng-ust-malloc to liblttng-ust-libc, update to lttng-ust 2.0

- includes a fix to tracepoints to disable the tracepoint sites upon
  unregistration of the library. This is useful for cases where, like
  the libc wrapper, the library dependency for constructor/destructor
  is not quite right due to cross-dependencies (lttng-ust depends on
  malloc/free). This ensures the session teardown will never be in a
  position where it could teardown data structures still in use
  by probes that cannot be unregistered because their associated
  library would already have called its destructor.

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