portlist: add macOS osImpl, finish migration to new style

Previously:

* 036f70b7b4 for linux
* 35bee36549 for windows

This does macOS.

And removes all the compat code for the old style. (e.g. iOS, js are
no longer mentioned; all platforms without implementations just
default to not doing anything)

One possible regression is that platforms without explicit
implementations previously tried to do the "netstat -na" style to get
open ports (but not process names). Maybe that worked on FreeBSD and
OpenBSD previously, but nobody ever really tested it. And it was kinda
useless without associated process names. So better off removing those
for now until they get a good implementation.

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

portlist/portlist_windows.go

@@ -14,11 +14,13 @@ import (
 	"tailscale.com/net/netstat"
 )
 
-// Forking on Windows is insanely expensive, so don't do it too often.
-const pollInterval = 5 * time.Second
-
 func init() {
 	newOSImpl = newWindowsImpl
+	// The portlist poller used to fork on Windows, which is insanely expensive,
+	// so historically we only did this every 5 seconds on Windows. Maybe we
+	// could reduce it down to 1 seconds like Linux, but nobody's benchmarked as
+	// of 2022-11-04.
+	pollInterval = 5 * time.Second
 }
 
 type famPort struct {
@@ -116,11 +118,3 @@ func procNameOfPid(pid int) string {
 	name = strings.TrimSuffix(name, ".EXE")
 	return name
 }
-
-func appendListeningPorts([]Port) ([]Port, error) {
-	panic("unused on windows; needed to compile for now")
-}
-
-func addProcesses([]Port) ([]Port, error) {
-	panic("unused on windows; needed to compile for now")
-}