lttng: list: print `per-user` and `per-process` buffer types

lttng: list: print `per-user` and `per-process` buffer types

The lttng-list command prints `per UID` and `per PID` for the
buffer type which is inconsistent with the terms used in the
documentation.

Moreover, `Buffer type` is replaced by `Buffering scheme` for
the same reason.

Change the print-out to match the online documentation.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id20098386ec0369fae017d01786ecc13c78820c0