all: Fix some linter errors (#5499)

I'm working through linter complaints, these are some fixes. Broad
categories:

1) Ignore errors where we can ignore errors: add "_ = ..." construct.
you can argue that this is annoying noise, but apart from silencing the
linter it *does* serve the purpose of highlighting that an error is
being ignored. I think this is OK, because the linter highlighted some
error cases I wasn't aware of (starting CPU profiles, for example).

2) Untyped constants where we though we had set the type.

3) A real bug where we ineffectually assigned to a shadowed err.

4) Some dead code removed.

There'll be more of these, because not all packages are fixed, but the
diff was already large enough.

lib/protocol/hello.go

@@ -77,12 +77,7 @@ func readHello(c io.Reader) (HelloResult, error) {
 		if err := hello.Unmarshal(buf); err != nil {
 			return HelloResult{}, err
 		}
-		res := HelloResult{
-			DeviceName:    hello.DeviceName,
-			ClientName:    hello.ClientName,
-			ClientVersion: hello.ClientVersion,
-		}
-		return res, nil
+		return HelloResult(hello), nil
 
 	case 0x00010001, 0x00010000, Version13HelloMagic:
 		// This is the first word of an older cluster config message or an