krpc: Add macros so that rpc.tlsservd can run in vnet prison

Commit 7344856e3a6d added a lot of macros that will front end
vnet macros so that nfsd(8) can run in vnet prison.
This patch adds similar macros named KRPC_VNETxxx so that
the rpc.tlsservd(8) daemon can run in a vnet prison, once the
macros front end the vnet ones. For now, they are null macros.

MFC after: 3 months