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.go

@@ -9,13 +9,16 @@ import (
 	"strings"
 )
 
+// Port is a listening port on the machine.
 type Port struct {
-	Proto   string
-	Port    uint16
-	inode   string
-	Process string
+	Proto   string // "tcp" or "udp"
+	Port    uint16 // port number
+	Process string // optional process name, if found
+
+	inode string // OS-specific; "socket:[165614651]" on Linux
 }
 
+// List is a list of Ports.
 type List []Port
 
 var protos = []string{"tcp", "udp"}
@@ -62,12 +65,12 @@ func (a List) SameInodes(b List) bool {
 }
 
 func (pl List) String() string {
-	out := []string{}
+	var sb strings.Builder
 	for _, v := range pl {
-		out = append(out, fmt.Sprintf("%-3s %5d %-17s %#v",
-			v.Proto, v.Port, v.inode, v.Process))
+		fmt.Fprintf(&sb, "%-3s %5d %-17s %#v\n",
+			v.Proto, v.Port, v.inode, v.Process)
 	}
-	return strings.Join(out, "\n")
+	return strings.TrimRight(sb.String(), "\n")
 }
 
 func GetList(prev List) (List, error) {