D37519 allows mountd(8)/nfsd(8) to optionally run in a vnet
prison. This patch allows gssd(8) to run in a vnet prison
as well, so that nfsd(8) can support Kerberized NFS mounts.

This patch completes the set required to support all NFS
mounts in a vnet prison.

Did Kerberized NFS mounts to nfsd(8) in vnet
prisons configured to only allow Kerberized mounts.
(ie sys=krb5)