API Fix: bt_ctf_iter_read_event_flags

The new bt_ctf_iter_read_event_flags function behaves like
bt_ctf_iter_read_event but takes a flag pointer.
This flag is used by the trace reading code to inform the user if some
events were discarded by the tracer in the current packet.

The new bt_ctf_get_lost_events_count function allows the user to get the
the number of events discarded immediately prior to the last event read.

[ Edit by Mathieu Desnoyers: use enum instead of define for
  BT_ITER_FLAG_LOST_EVENTS. ]

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