kern_jail.c: Allow mountd/nfsd to optionally run in a jail

This patch adds "allow.nfsd" to the jail code based on a
new kernel build option VNET_NFSD. This will not work
until future patches fix nmount(2) to allow mountd to
run in a vnet prison and the NFS server code is patched
so that global variables are in a vnet.

The jail(8) man page will be patched in a future commit.

Reviewed by: jamie
MFC after: 4 months
Differential Revision: https://reviews.freebsd.org/D37637