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