runtime: report correct error if kevent failed in netpollinit

Report the value returned by kevent, not the previously set errno which
is 0.

Found while debugging CL 198544

Change-Id: I854f5418f8ed8e083d909d328501355496c67a53
Reviewed-on: https://go-review.googlesource.com/c/go/+/202777
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/netpoll_kqueue.go

@@ -35,7 +35,7 @@ func netpollinit() {
  *(*uintptr)(unsafe.Pointer(&ev.ident)) = uintptr(r)
  n := kevent(kq, &ev, 1, nil, 0, nil)
  if n < 0 {
- println("runtime: kevent failed with", -errno)
+ println("runtime: kevent failed with", -n)
  throw("runtime: kevent failed")
  }
  netpollBreakRd = uintptr(r)