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.

cmd/syncthing/usage.go

@@ -19,11 +19,11 @@ func optionTable(w io.Writer, rows [][]string) {
 	for _, row := range rows {
 		for i, cell := range row {
 			if i > 0 {
-				tw.Write([]byte("\t"))
+				_, _ = tw.Write([]byte("\t"))
 			}
-			tw.Write([]byte(cell))
+			_, _ = tw.Write([]byte(cell))
 		}
-		tw.Write([]byte("\n"))
+		_, _ = tw.Write([]byte("\n"))
 	}
 	tw.Flush()
 }