posix_spawn(2) on macOS

The text 427c1bf354e4, as macOS 10.12.6, macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

spawn a process

posix_spawn(2) on macOS now