spec, libcni, pkg/invoke: Use OS-agnostic separator when parsing CNI_PATH

Hardcoding the list separator character as ":" causes CNI to fail when parsing
CNI_PATH on other operating systems. For example, Windows uses ";" as list
separator because ":" can legally appear in paths such as "C:\path\to\file".
This change replaces use of ":" with OS-agnostic APIs or os.PathListSeparator.

Fixes #358

libcni/api.go

@@ -15,6 +15,7 @@
 package libcni
 
 import (
+	"os"
 	"strings"
 
 	"github.com/containernetworking/cni/pkg/invoke"
@@ -165,6 +166,6 @@ func (c *CNIConfig) args(action string, rt *RuntimeConf) *invoke.Args {
 		NetNS:       rt.NetNS,
 		PluginArgs:  rt.Args,
 		IfName:      rt.IfName,
-		Path:        strings.Join(c.Path, ":"),
+		Path:        strings.Join(c.Path, string(os.PathListSeparator)),
 	}
 }