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/monitor_test.go

@@ -17,7 +17,7 @@ import (
 func TestAutoClosedFile(t *testing.T) {
 	os.RemoveAll("_autoclose")
 	defer os.RemoveAll("_autoclose")
-	os.Mkdir("_autoclose", 0755)
+	_ = os.Mkdir("_autoclose", 0755)
 	file := filepath.FromSlash("_autoclose/tmp")
 	data := []byte("hello, world\n")