Use gofumpt to format code

gofumpt (mvdan.cc/gofumpt) is a fork of gofmt with stricter rules.

Brought to you by

	git ls-files \*.go | grep -v ^vendor/ | xargs gofumpt -s -w

Looking at the diff, all these changes make sense.

Also, replace gofmt with gofumpt in golangci.yml.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

spec.go

@@ -111,7 +111,7 @@ created by an unprivileged user.
 		if err != nil {
 			return err
 		}
-		return ioutil.WriteFile(specConfig, data, 0666)
+		return ioutil.WriteFile(specConfig, data, 0o666)
 	},
 }