krpc: Allow mountd/nfsd to optionally run in a jail

This patch modifies the kernel RPC so that it will allow
mountd/nfsd to run inside of a vnet jail. Running mountd/nfsd
inside a vnet jail will be enabled via a new kernel build
option called VNET_NFSD, which will be implemented in future
commits.

Although I suspect cr_prison can be set from the credentials
of the current thread unconditionally, I #ifdef'd the code
VNET_NFSD and only did this for the jailed case mainly to
document that it is only needed for use in a jail.

The TLS support code has not yet been modified to work in
a jail. That is planned as future development after the
basic VNET_NFSD support is in the kernel.

This patch should not result in any semantics change until
VNET_NFSD is implemented and used in a kernel configuration.

MFC after: 4 months