Decompose system/file.go for POSIX/Windows to allow building for Windows

[eripa]

Note: The minor changes here was done to allow for an internal specific use case, I'm just putting this here in case someone wants to pick it up. Personally I probably won't be working more on this.

This change enables goss to build for Windows. Initial step for #191.

Some caveats:

• a test suite hasn't been created
  • cannot really use Docker for this, maybe Vagrant in the future?
  • this means that there are probably many bugs that I haven't seen
• I've only tested it on Windows 8.1, using cygwin x86_64
  • which also means that there are probably many more issues on "pure" Windows
• the Mode, Owner and Group function of system/file.go has been mocked with dummy data to build on Windows. Ideally these should be implemented in some platform native manner (that I'm not familiar with)

Example from my usecase:

vagrant@test_goss_windows ~
$ ./goss-windows.exe -g goss.yaml validate
.........

Total Duration: 0.033s
Count: 9, Failed: 0, Skipped: 0

vagrant@test_goss_windows ~
$ cat goss.yaml
file:
  C:\cygwin64\home\vagrant\some_file_that_doesnt_exist:
    exists: false
    contains: []
  C:\cygwin64\home\vagrant\consul.exe:
    exists: true
    mode: "0000"
    size: 43419136
    owner: Other Organization
    group: Other Organization
    filetype: file
    contains: []
command:
  /home/vagrant/disk_usage.exe -warning 50 -critical 60:
    exit-status: 0
    stdout: []
    stderr: []
    timeout: 10000
process:
  consul.exe:
    running: true

vagrant@test_goss_windows ~
$ uname -a
CYGWIN_NT-6.3 test_goss_windows 2.0.4(0.287/5/3) 2015-06-09 12:22 x86_64 Cygwin

[JackLeo]

what about other parts of the tool? ports, services, etc?

[petemounce]

Packages (check via chocolatey)?

[aelsabbahy]

closed by #585