Unfortunately, MacOS doesn't provide sys/tree.h -- so the compat check for this must happen portably.

This is searched for portably.

For the future, this will make it possible to provide alternative implementations of functions now stored in object_open_privsep.c. This will probably be needed by future gotd(8) which runs inside a chroot(2) environment and without the "exec" pledge(2) promise, making it impossible to run libexec helpers on the fly. Details of this design are not yet settled, but moving functions into a separate compilation unit won't hurt in any case.