nvmf_proto.h: NVMe over Fabrics protocol definitions

This is a copy of spdk/include/spdk/nvmf_spec.h as of commit
470e851852bb948334a272c9f8de495020fa082f from Intel's SPDK.
Subsequent commits will modify it to be suitable header for the
kernel, but importing the stock file first makes it easier to see
how the resulting header is derived from the original.

Reviewed by: imp
Obtained from: SPDK (https://github.com/spdk/spdk.git)
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44703