Callbacks on receive and update FD

The user of the lib can now take control over a new FD or the update
operation of an existing FD.
Opening the output tracefile is now the responsiblity of the user
and not the library itself.

[ Edit by Mathieu Desnoyers: comment and teardown cleanups ]

Signed-off-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>