sys: Remove $FreeBSD$: two-line .h pattern

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spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD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

powerpc: clean up empty lines in .c and .h files

sys/powerpc: 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.

Fix the deciKelvin to Celsius conversion in kernel.

After r285994, sysctl(8) was fixed to use 273.15 instead of 273.20 as 0C
reference and as result, the temperature read in sysctl(8) now exibits a
+0.1C difference.

This commit fix the kernel references to match the reference value used in
sysctl(8) after r285994.

Sponsored by: Rubicon Communications (Netgate)

- Introduce a define for ZERO_C_TO_K.
- Fix the printing of the temperature when we exceed the critical value.

Approved by: nwhitehorn (mentor)

Factor out the SMU fan management code into a new module (powermac_thermal)
that will connect all of the various sensors and fan control modules on
Apple hardware with software-controlled fans (e.g. all G5 systems).

MFC after: 1 month