Allow lttng_directory_handle to be moved

Implement a "move" method (akin to C++'s move semantics) which allows
the transfer of the contents/ownership of a stack-allocated directory
handle to another object.

A moved directory handle must no longer be used after the "move"
operation. The original moved handle is invalidated in the hope
of catching erroneous uses of this API quickly.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>