audit: do not complain if kernel lacks audit

audit: do not complain if kernel lacks audit

When running on a kernel without audit support, systemd currently
writes a mysterious-sounding error to its log:

	systemd[1]: Failed to connect to audit log: Protocol not supported

Better to suppress the audit_open() failure message when (and only
when) it is due to running on a kernel without audit support, since in
this case the admin probably does not mind systemd not writing to the
audit log.  This way, more serious errors like ENOMEM and EACCES will
stand out more.