fchflags(2) on macOS

The text d29fc276a044, as macOS 10.15.6, macOS 10.15.5, macOS 10.15.4, macOS 10.15.3, macOS 10.15.2, macOS 10.15.1, macOS 10.15 shipped it.

set file flags

fchflags(2) on macOS now