portlist: update some internals to use append-style APIs

In prep for reducing garbage, being able to reuse memory.  So far this
doesn't actually reuse much. This is just changing signatures around.

But some improvement in any case:

    bradfitz@tsdev:~/src/tailscale.com$ ~/go/bin/benchstat before after
    name       old time/op    new time/op    delta
    GetList-8    11.8ms ± 9%     9.9ms ± 3%  -15.98%  (p=0.000 n=10+10)

    name       old alloc/op   new alloc/op   delta
    GetList-8    99.5kB ± 2%    91.9kB ± 0%   -7.62%  (p=0.000 n=9+9)

    name       old allocs/op  new allocs/op  delta
    GetList-8     3.05k ± 1%     2.93k ± 0%   -3.83%  (p=0.000 n=8+9)

More later, once parsers can reuse strings from previous parses.

Updates #5958

Change-Id: I76cd5048246dd24d11c4e263d8bb8041747fb2b0
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

portlist/portlist_linux_test.go

@@ -132,8 +132,10 @@ func BenchmarkParsePorts(b *testing.B) {
 
 func BenchmarkListPorts(b *testing.B) {
 	b.ReportAllocs()
+	var base []Port
 	for i := 0; i < b.N; i++ {
-		_, err := listPorts()
+		var err error
+		base, err = appendListeningPorts(base[:0])
 		if err != nil {
 			b.Fatal(err)
 		}