selinux: when dropping capabilities only include AUDIT caps if we have them

selinux: when dropping capabilities only include AUDIT caps if we have them

When we drop capabilities we shouldn't assume we can keep
CAP_AUDIT_WRITE unconditionally, since it will not be available when
running in containers.

This patch only adds CAP_AUDIT_WRITE to the list of caps we keep if we
actually have it in the first place.

This makes audit/selinux enabled D-Bus work in a Linux container.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=49062
Acked-by: Thiago Macieira <thiago@kde.org>
Acked-by: Colin Walters <walters@verbum.org>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>