Port slave/slave.cpp to Windows

Depends on pretty much all "porting" tasks in the slave component. COmplete those first.

And impartial dependency graph looks like this:

1. async: dispatch, future, process
2. address.hpp: *, ip.hpp, net.hpp
3. check: future
4. containerizer: future, process, fetcher, flags
5. collect: future, defer, process
6. defer.hpp: deferred.hpp, dispatch.hpp
7. deferred.hpp: dispatch.hpp, pid.hpp
8. delay.hpp: dispatch.hpp
9. disk: flags, state, isolator
10. dispatch.hpp: process.hpp
11. executor.hpp: defer.hpp, deferred.hpp, process.hpp
12. fetch.hpp: path.hpp
13. fetcher: type_utils, future, process, subprocess
14. flags.hpp: os.hpp, path.hpp, fetch.hpp
15. fs: *
16. future.hpp: pid.hpp
17. isolator: process
18. ip.hpp: *
19. launch: subcommand
20. launcher: isolator, future, usbprocess, flags
21. monitor.hpp: future.hpp
22. net.hpp: ip, networking stuff
23. noop: resource_estimator
24. path.hpp: *
25. pid.hpp: address.hpp, ip.hpp
26. process.hpp: *
27. protobuf.hpp: *
28. protobuf_utils: isolator, ip, uuid
29. filesystem/posix: flags, isolator
30. os.hpp: *
31. reap: future
32. resource_estimator: future
33. state: type_utils, pid, os, path, protobuf, uuid
34. subcommand: flags
35. subprocess: os, future
36. type_utils.hpp: uuid.hpp
37. uuid.hpp: *