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/perfstats_unix.go

@@ -38,7 +38,10 @@ func savePerfStats(file string) {
 
 	t0 := time.Now()
 	for t := range time.NewTicker(250 * time.Millisecond).C {
-		syscall.Getrusage(syscall.RUSAGE_SELF, &rusage)
+		if err := syscall.Getrusage(syscall.RUSAGE_SELF, &rusage); err != nil {
+			continue
+		}
+
 		curTime := time.Now().UnixNano()
 		timeDiff := curTime - prevTime
 		curUsage := rusage.Utime.Nano() + rusage.Stime.Nano()