Fix context API

- bt_context_add_traces_recursive ->
  don't have 2 symbols that just differ from a "s".
- bt_context_add_trace return values:
  - Now, nonzero is error, zero is OK.
  - Streamline with bt_context_add_traces_recursive, so none return
    trace handle IDs.

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