Port TrueNAS NFSv4 support (commit dded53703496a0e2b942fae19a7e2612d8a3b616)
to FreeNAS:

Add an easter egg style basic NFSv4 support.  The user have to manually
touch /conf/base/etc/nfsv4_enable and reboot (or touch /etc/nfsv4_enable)
to enable it for now.

Ticket: #3582