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.

style: is-valid-shell-script: skip filtered files

If a file is filtered by .stylefilter, that means it's probably not
owned by us - and so we shouldn't check the validity of it if it is
actually a shell script.

Rename "posix" -> "valid"

The original plan was to have two scripts, one which checked for
POSIX-ness and one which ran the internal syntax checker.

But...

The shell function I wrote to tell if the operand was a shell script
dominated the file and I didn't want to repeat it or source it in.

So the script performs both tests.