SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
And also mark strlcpy() and strlcat() with __attribute__((warn_unused_result)).
Since our code is warning-free, this ensures we never misuse those functions.
(Or are very sure about doing it when turning off the warning for a particular
piece of code.)
