rewrite html generator with html/template

internal/disk_usage/disk_usage_unix.go

@@ -0,0 +1,24 @@
+//go:build linux || darwin || freebsd
+
+// "_unix" is not a special file name suffix for Go, so the OS list is in the //go:build line.
+// Not "unix": openbsd and netbsd have different Statfs_t or no syscall.Statfs.
+
+package disk_usage
+
+import "syscall"
+
+func diskUsage(path string) (Usage, error) {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return Usage{}, err
+	}
+	// Field types differ between OSes, int64() works for all of them.
+	bsize := int64(st.Bsize)
+	return Usage{
+		Total: int64(st.Blocks) * bsize,
+		// Bfree: all free blocks, including reserved for root
+		Used: (int64(st.Blocks) - int64(st.Bfree)) * bsize,
+		// Bavail: free blocks available to unprivileged users
+		Free: int64(st.Bavail) * bsize,
+	}, nil
+}