Add new port for Linux-nvme's nvme-cli program.

The nvme program allows sending of arbitrary commands to nvme devices.
Only the admin-passthru command is known to work, though others might.
Many linuxisms remain in this program that, though it compiles, will
likely not work on FreeBSD, preventing other commands from working.
Marked only for amd64 / FreeBSD 12/13 since it's only been tested
there and uses APIs that are unstable between the major releases.
Upstreamable patches welcome to fix any of this, and this port will
evolve as my patches get accepted upstream.