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.
Remove unneeded `vproc_swap_integer` calls
Remove `#include <bootstrap.h>`

`bootstrap_port` is also declared in `mach/mach_init.h`, so this is
not necessary.
Move the actual implementation into functions with a dedicated file
Mark OS X 10.10 as a supported version
test: Add diagnostic for `move-to-user=10.10`
Merge branch 'lembacon/yosemite'