Remove $FreeBSD$: one-line sh pattern

Remove /^\s*#[#!]?\s*\$FreeBSD\$.*$\n/

spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

Ensure `kyua list` working when there is no /dev/nvme*

Sponsored by: The FreeBSD Foundation

Add a basic test for nvmecontrol

I recently made some bug fixes in nvmecontrol. It occurred to me that
since nvmecontrol lacks any kyua tests, I should convert the informal
testing I did into a more formal automated test. The test in this
change should be considered just a starting point; it is neither
complete nor thorough. While converting the test to ATF/kyua, I
discovered a small bug in nvmecontrol; the nvmecontrol devlist command
would always exit with an unsuccessful status. So I included the fix
for that, too, so that the test won't fail.

Reviewed by: imp@
MFC after: 3 days
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24269