fchflags(2) on macOS

The text 0c1308e0dd8f, as macOS 12.2, macOS 12.1, macOS 12.0.1, macOS 11.5, macOS 11.4, macOS 11.3, macOS 11.2, macOS 11.1, macOS 11.0.1 shipped it.

set file flags