all: Become a Go module (fixes #5148) (#5384)

* go mod init; rm -rf vendor

* tweak proto files and generation

* go mod vendor

* clean up build.go

* protobuf literals in tests

* downgrade gogo/protobuf

vendor/github.com/calmh/du/README.md

@@ -0,0 +1,14 @@
+du
+==
+
+Get total and available disk space on a given volume.
+
+Documentation
+-------------
+
+http://godoc.org/github.com/calmh/du
+
+License
+-------
+
+Public Domain

vendor/github.com/calmh/du/cmd/du/main.go

@@ -1,21 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"log"
-	"os"
-
-	"github.com/calmh/du"
-)
-
-var KB = int64(1024)
-
-func main() {
-	usage, err := du.Get(os.Args[1])
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Free:", usage.FreeBytes/(KB*KB), "MiB")
-	fmt.Println("Available:", usage.AvailBytes/(KB*KB), "MiB")
-	fmt.Println("Size:", usage.TotalBytes/(KB*KB), "MiB")
-}