Add lib/diskusage to measure used/free on disks

lib/diskusage/diskusage_unsupported.go

@@ -0,0 +1,10 @@
+//go:build illumos || js || plan9 || solaris
+
+package diskusage
+
+// New returns the disk status for dir.
+//
+// May return Unsupported error if it doesn't work on this platform.
+func New(dir string) (info Info, err error) {
+	return info, ErrUnsupported
+}