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/nat/service.go

@@ -92,7 +92,7 @@ func (s *Service) process() int {
 			toRenew = append(toRenew, mapping)
 		} else {
 			toUpdate = append(toUpdate, mapping)
-			mappingRenewIn := mapping.expires.Sub(time.Now())
+			mappingRenewIn := time.Until(mapping.expires)
 			if mappingRenewIn < renewIn {
 				renewIn = mappingRenewIn
 			}
@@ -328,6 +328,6 @@ findIP:
 
 func hash(input string) int64 {
 	h := fnv.New64a()
-	h.Write([]byte(input))
+	_, _ = h.Write([]byte(input))
 	return int64(h.Sum64())
 }