Enable Windows CI (Appveyor)

- start list of linux_only plugins; ignore them when testing on Windows
- Isolate linux-only code by filename suffix
- Remove stub (NotImplemented) functions
- other misc. fixes for Windows compatibility

.appveyor.yml

@@ -0,0 +1,28 @@
+clone_folder: c:\gopath\src\github.com\containernetworking\plugins
+
+environment:
+  GOPATH: c:\gopath
+
+install:
+  - echo %PATH%
+  - echo %GOPATH%
+  - set PATH=%GOPATH%\bin;c:\go\bin;%PATH%
+  - go version
+  - go env
+
+build: off
+
+test_script:
+  - ps: |
+      go list ./... | Select-String -Pattern (Get-Content "./plugins/linux_only.txt") -NotMatch > "to_test.txt"
+      echo "Will test:"
+      Get-Content "to_test.txt"
+      foreach ($pkg in Get-Content "to_test.txt") {
+        if ($pkg) {
+          echo $pkg
+          go test -v $pkg
+          if ($LastExitCode -ne 0) {
+            throw "test failed"
+          }
+        }
+      }