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_other.go

@@ -6,8 +6,10 @@
 
 package portlist
 
+import "time"
+
 // We have to run netstat, which is a bit expensive, so don't do it too often.
-const POLL_SECONDS = 5
+const pollInterval = 5 * time.Second
 
 func listPorts() (List, error) {
 	return listPortsNetstat("-na")