Extract mknod, umask, lstat to pkg/system

Some parts of pkg/archive is called on both client/daemon code. To get
it compiling on Windows, these funcs are extracted into files with
build tags.

Signed-off-by: Ahmet Alp Balkan <ahmetb@microsoft.com>

system/umask.go

@@ -0,0 +1,11 @@
+// +build !windows
+
+package system
+
+import (
+	"syscall"
+)
+
+func Umask(newmask int) (oldmask int, err error) {
+	return syscall.Umask(newmask), nil
+}