Replace is_win function by IS_WINDOWS flag.

Use os.name instead of sys.platform (more generic).
Fix get_maxfd in circusd on Windows
Disable daemonizing on Windows

Spawning a daemon on Windows is kinda hard. For now,
we don't support it, but someone with Windows knowledge
should be able to add it.
Accept dev versions of ZMQ in circusd
Disable rlimit commands on unsupported platforms
Don't use preexec function in Process on Windows

Unfortunatly, Windows does not support the pre-exec
function in Popen (psutil does not either).
As most of this function's work is to set Unix related
stuff (gid, rlimits...) it should not be an issue.
Refactor Pidfile.create. Fix it on Windows.
use_fds must be True on Windows when piping
Disable Streams on Windows

Tornado does not handle listening for events on
non-sockets fds. Hence, we can't use Streaming on
Windows.
This could be fixed by adding IOCP compatibility to
Tornado.
Refactor sighandler.py. Add Windows support.
Disable AF_UNIX sockets on unsupported platforms.
Don't send SIGKILL on Windows.
Set copy_env to True by default on Windows

On Windows, most programs can't work with an empty env.
However, instead of setting copy_env to True, we could
eventually set-up a minimal env instead.
Handle AccessDenied when calling Process.terminate
Use posix=False on Windows with shlex.split
Don't use a WatchedFileHandler on Windows
Refactor Watcher.reap_process for Windows
Don't wait for children in the Arbiter on Windows

I'm not sure if we could do something similar on
Windows.
Calling waitpid with a negative PID has no
meaning on this platform, so we would have to use
an alternative.
However, I'm not sure if orphans are an issue.
Fix ZeroMQ assertion error in the client.

The client was closing its socket instead of closing
the streamer. On Windows, if any data was sent to the
socket after that, ZeroMQ was aborting with "Socket
operation on non-socket".
Add PYTHON helper in tests/support.py.

This helper should be used instead of sys.executable.