Add lib/diskusage to measure used/free on disks

lib/diskusage/diskusage.go

@@ -0,0 +1,15 @@
+// Package diskusage provides a cross platform version of the statfs
+// system call to read disk space usage.
+package diskusage
+
+import "errors"
+
+// Info is returned from New showing details about the disk.
+type Info struct {
+	Free      uint64 // total free bytes
+	Available uint64 // free bytes available to the current user
+	Total     uint64 // total bytes on disk
+}
+
+// ErrUnsupported is returned if this platform doesn't support disk usage.
+var ErrUnsupported = errors.New("disk usage unsupported on this platform")