Fix: lttng_directory_handle_init fails on opening base relayd output

lttng_directory_handle_init, called from session_set_anonymous_chunk,
fails to open "$LTTNG_HOME/lttng-traces" directory as the folder is
not yet created at this time.

Rename init_session_output_directory_handle to
session_init_output_directory_handle and move it to session.h

For an anonymous chunk, the session->output_path is empty.  The
resulting output directory handle is at the "root" node of the
lttng-relayd chosen output.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>