portlist: document, clean up, fix an open fd spike, optimize a bit

I noticed portlist when looking at some profiles and hadn't looked at
the code much before. This is a first pass over it. It allocates a
fair bit. More love remains, but this does a bit:

name       old time/op    new time/op    delta
GetList-8    9.92ms ± 8%    9.64ms ±12%     ~     (p=0.247 n=10+10)

name       old alloc/op   new alloc/op   delta
GetList-8     931kB ± 0%     869kB ± 0%   -6.70%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
GetList-8     4.59k ± 0%     3.69k ± 1%  -19.71%  (p=0.000 n=10+10)

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

portlist/portlist_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package portlist
+
+import "testing"
+
+func TestGetList(t *testing.T) {
+	pl, err := GetList(nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	for i, p := range pl {
+		t.Logf("[%d] %+v", i, p)
+	}
+	t.Logf("As String: %v", pl.String())
+}
+
+func BenchmarkGetList(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_, err := GetList(nil)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}