portlist: unexport all Poller fields, removing unused one, rework channels

Poller.C and Poller.c were duplicated for one caller. Add an accessor
returning the receive-only version instead. It'll inline.

Poller.Err was unused. Remove.

Then Poller is opaque.

The channel usage and shutdown was a bit sketchy. Clean it up.

And document some things.

Change-Id: I5669e54f51a6a13492cf5485c83133bda7ea3ce9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -1530,7 +1530,7 @@ func dnsMapsEqual(new, old *netmap.NetworkMap) bool {
 func (b *LocalBackend) readPoller() {
 	n := 0
 	for {
-		ports, ok := <-b.portpoll.C
+		ports, ok := <-b.portpoll.Updates()
 		if !ok {
 			return
 		}