bus: move shared libaudit code to a new audit.[ch]

bus: move shared libaudit code to a new audit.[ch]

This fixes various duplicated libaudit interactions in both
SELinux and AppArmor code paths, including opening two audit sockets
if both SELinux and AppArmor were enabled at compile time.
In particular, audit.c is now the only user of libcap-ng.

This commit is not intended to introduce any functional changes,
except for the de-duplication.

The actual audit_log_user_avc_message() call is still duplicated,
because the SELinux and AppArmor code paths use different mechanisms
to compose the audit message: the SELinux path uses a statically-sized
buffer on the stack which might be subject to truncation, whereas
the AppArmor path uses malloc() (via DBusString) and falls back to
using syslog on a memory allocation failure.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89225
Reviewed-by: Colin Walters <walters@verbum.org>
[smcv: minor issues raised during review are subsequently fixed]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>