use SPDX short-form identifiers for license info in code files

SPDX short-form identifiers are the new preferred way to specify license info inside of code files. They are short, and both human and machine readable.

The license/copyright info would then simply be:

# SPDX-License-Identifier: GPL-2.0-or-later
# Copyright (C) Evan Goetz (2022)

This is a very low priority issue.