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 *
When receiving a file descriptor over IPC, the receiver must now call
take_fd() on the IPC::File to take over the descriptor. Otherwise,
IPC::File will close the file on destruction.
