Add NVMe Namespace Management support to nvmecontrol(8).

This allows create/delete/attach/detach namespaces on new NVMe controllers.
This is only a first user-level part of the bigger change set. Kernel part
required to detect and handle the configuration changes without reboot is
completely independent and will be added separately.

Submitted by: Matt Williams <mffbsdw@gmail.com> (original version)
Differential Revision: https://reviews.freebsd.org/D11399