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/mknod.go

@@ -0,0 +1,18 @@
+// +build !windows
+
+package system
+
+import (
+	"syscall"
+)
+
+func Mknod(path string, mode uint32, dev int) error {
+	return syscall.Mknod(path, mode, dev)
+}
+
+// Linux device nodes are a bit weird due to backwards compat with 16 bit device nodes.
+// They are, from low to high: the lower 8 bits of the minor, then 12 bits of the major,
+// then the top 12 bits of the minor
+func Mkdev(major int64, minor int64) uint32 {
+	return uint32(((minor & 0xfff00) << 12) | ((major & 0xfff) << 8) | (minor & 0xff))
+}