Make nvd vs nda choice boot-time rather than build-time

Introduce hw.nvme.use_nvd tunable. This tunable allows both nvd and
nda to be installed in the kernel, while allowing only one of them to
create devices. This is an all-or-nothing setting, and you can't
change it after boot-time. However, it will allow easier A/B testing.

Differential Revision: https://reviews.freebsd.org/D11825