nvmfdd: A simple userspace NVMe over Fabrics host

This program uses libnvmf to connect to a remote Fabrics controller
and perform a single read or write operation. The write command reads
data from stdin to construct one or more NVM Write commands sent to
the remote namespace. The read command uses one or more NVM Read
commands to read blocks from a remote namespace writing the data to
stdout.

Reviewed by: chuck, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44730