Provide unconditionally compiled stub implementations of audit system

Provide unconditionally compiled stub implementations of audit system
calls that simply return ENOSYS.  This will allow basic forward
compatibility with userland audit pieces to be committed in the future,
avoiding the "login killed by SIGSYS" problem users of CVS HEAD
experienced when booting older kernels with newer user spaces (an
unsupported but not uncommon situation when debugging problems).

Approved by:	re (scottl)
Obtained from:	TrustedBSD Project