sys: Remove $FreeBSD$: one-line .c comment pattern

Remove /^/[*/]\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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

sys/dev: further adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

Use new OSS-based BSD-licensed header for cs sound driver.

The cs driver requires a table with firmware values. An
alternative firmware is available in a similar Open Sound
System driver. This is actually a partial revert of
Revision 77504.

Special thanks to joel@ for patiently testing several
replacement attempts.

The csa driver and the complete sound system are now free
of the GPL.

Tested by: joel
Approved by: jhb (mentor)
MFC after: 3 weeks