src.ctf.fs: make ctf_fs_file_create return a unique_ptr

src.ctf.fs: make ctf_fs_file_create return a unique_ptr

Introduce ctf_fs_file::UP, make ctf_fs_file_create return that, adjust
impacted callers.

Change-Id: Ice241fd596af3d54e9c7bc7f562ca010bb5833aa
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8260
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12298