Cleanup: separate unix-related codes to make Swarm buildable on Windows

[chanwit]

I am new to contributing to Docker projects. Let me know if I am doing something wrong.

There will be some move to make Docker and its related projects available for Windows in the new future. Swarm is still young so it's great to have some Windows-related refactoring early for this project.

This PR is initially separating Unix / Windows specific codes into different files and guarding them with the platform +build flag. This PR also allows contributors to compile and start hacking Swarm on Windows.

[vieux]

you can use // +build !windows here

[chanwit]

@vieux no problem, I'll make a change.

[vieux]

ping @icecrime, could you please test this

[icecrime]

icecrime@icecrime-PC /cygdrive/e/src/github.com/docker/swarm
$ go build

icecrime@icecrime-PC /cygdrive/e/src/github.com/docker/swarm
$ /cygdrive/e/bin/swarm.exe
NAME:
   swarm - docker clustering

USAGE:
   swarm [global options] command [command options] [arguments...]

VERSION:
   0.0.1

COMMANDS:
   create, c    create a cluster
   list, l      list nodes in a cluster
   manage, m    manage a docker cluster
   join, j      join a docker cluster
   help, h      Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --debug              debug mode [$DEBUG]
   --help, -h           show help
   --version, -v        print the version

👍

[chanwit]

updated to use // +build !windows in api/server_unix.go per comment from @vieux

[aluzzardi]

LGTM