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 *
This was a helper that would call a syscall repeatedly until it either
succeeded or failed with a non-EINTR error.
It was only used in two places, so I don't think we need this helper.
