Update cloexec.* vendored files

These files are only used by the main program. Therefore, delete them from common, and add them to gnulib to emphasize they are vendored.

Update to the latest version in the git repository, as of 2023-06-18.

Note that the header file has been changed into a C23 file, minimal changes have been applied for C99 compatibility.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Documentation updated / NEWS entry present (for non-trivial changes)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code