Fix: ust: keep using lttng-ust-sock-8 and lttng-ust-wait-8 filenames

Fix: ust: keep using lttng-ust-sock-8 and lttng-ust-wait-8 filenames

Replicate a change from a lttng-ust header for which there is an
internal copy within lttng-tools for building without lttng-ust.

lttng-ust commit 6a359b8a4006 ("Bump LTTNG_UST_ABI version from 8.1 to
9.0") attempted to ensure that a session daemon linked against an old
lttng-ust-ctl (2.12) would not attempt to interact with newer
applications.

This was done by increasing the major ABI version number, but
introducing LTTNG_UST_ABI_MAJOR_VERSION_OLDEST_COMPATIBLE (set to 8) as
a mean to allow session daemon to accept both major versions 8 and 9.

Unfortunately, changing LTTNG_UST_ABI_MAJOR_VERSION means the filenames
used for communication changed as well, meaning that applications linked
against older lttng-ust 2.12 cannot interact with lttng sessiond linked
against lttng-ust-ctl 2.13, which is unintended.

Therefore, go back to using the previous filenames: lttng-ust-sock-8,
and lttng-ust-wait-8 for communication.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I121c6e5aa13637f13b996dd9e31e06d060b52ca6