portlist: populate Pid field on Linux

The Port.Pid was always more of an implementation detail on some
platforms and isn't necessary on Linux so it was never populated.
(Nothing outside the portlist package ever used it)

But might as well populate it for consistency since we have it in
memory and its absence confused people.

Updates #cleanup

Change-Id: I869768a75c9fedeff242a5452206e2b2947a17cb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

portlist/portlist_linux.go

@@ -322,6 +322,8 @@ func (li *linuxImpl) findProcessNames(need map[string]*portMeta) error {
 				pe.pid = int(p)
 			}
 			pe.port.Process = argvSubject(argv...)
+			pid64, _ := mem.ParseInt(pid, 10, 0)
+			pe.port.Pid = int(pid64)
 			pe.needsProcName = false
 			delete(need, string(targetBuf[:n]))
 			if len(need) == 0 {