kgssapi: Add macros so that gssd(8) 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 KGSS_VNETxxx so that
the gssd(8) daemon can run in a vnet prison, once the
macros front end the vnet ones. For now, they are null macros.

This is the last commit that adds macros. The next step is
to change the macros to front end the vnet ones.

MFC after: 3 months