trivial: Always invoke tools with python3

Signed-off-by: Curtis Millar <curtis.millar@data61.csiro.au>

checking pragma for kernel header files

lifting the gurad of checking pragma in kernel's header files.
Details:
All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

Make style cleanups.

* misc/style-c.sh: Use the shell's && instead of "test -a", which is now
"obsolescent" per POSIX 2018 (see
http://pubs.opengroup.org/onlinepubs/9699919799.2018edition/).

* misc/style-all.sh: Indent branches of if-then-else conditional.

* misc/style-c.sh:
* misc/style-py.sh: [English] Prefer "in place" to "in-place" when not a
attributive modifier.

* misc/style-changed.sh:
* misc/style.sh: [English] Give comment sentences terminal punctuation.

* misc/cobbler:
* misc/style-c.sh:
* misc/style-cmake.sh:
* misc/style-py.sh:
* misc/style.sh: Standardize whitespace around CSIRO header.

misc: fix bug in style-c.sh

Use and instead of or to avoid styling kernel with guardonce.

fix: check against 'seL4' when doing guardonce

misc/style-c.sh: Add guardonce style

This check replaces header guards with #pragmaonce, unless the repo is
called "kernel" as we cannot use header guards in the kernel.

misc: increase functionality of style.sh

- split style.sh into style-c.sh and style-cmake.sh
- add style.py, which passes files based on extension to the correct
style tools and ignores files specified in a .stylefilter file (only at
the top level)
- style.sh now calls style.py and expects a list of files
- add style-changed.sh, which styles all changed files in a git repo.
- add style-all.sh, which styles all files in directories passed to it