Fix for OS X Yosemite 10.10

On OS X 10.10, the original `_vprocmgr_move_subset_to_user` has been
moved from `liblaunch.dylib` to `libxpc.dylib`, which always returns
`XPC_BOOL_FALSE`.

After some nasty reverse engineering, and some study of the source code
of `launchd` (the version matches the one used in OS X 10.9.2) which is
available from `http://opensource.apple.com`, I ripped some snippets off
there and made this up, and it works.