This changes Core::File::open() to specify O_CLOEXEC by default
so that we don't leak file descriptors into child processes. The
new behavior can be overriden by specifying OpenMode::KeepOnExec.
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 *
