move code which forks child processes into a subroutine, reducing copy-pasta

move functions which open objects into new file object_open_privsep.c 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.