basic/audit-util: make a test request before enabling use of audit

basic/audit-util: make a test request before enabling use of audit

If a container manager does not follow the guidance in
https://systemd.io/CONTAINER_INTERFACE/ regarding audit capabilities,
then the current check may not be sufficient to determine that audit
will function properly. In particular, when calling bind() on the audit
fd, we will get EPERM if running in a user-namespaced container.

Expand the check to make an AUDIT_GET_FEATURE request on the audit fd to
test if it is working. If this fails with ECONNREFUSED, we know it is
because the kernel does not support the use of audit outside of the
initial user namespace.

Note that the approach of this patch was suggested here:
https://github.com/systemd/systemd/pull/19443#issuecomment-829566659

Fixes: #6519