hostinfo, ipnlocal: flesh out Wake-on-LAN support, send MACs, add c2n sender

This optionally uploads MAC address(es) to control, then adds a
c2n handler so control can ask a node to send a WoL packet.

Updates #306

RELNOTE=now supports waking up peer nodes on your LAN via Wake-on-LAN packets

Change-Id: Ibea1275fcd2048dc61d7059039abfbaf1ad4f465
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

hostinfo/hostinfo.go

@@ -57,6 +57,7 @@ func New() *tailcfg.Hostinfo {
 		Cloud:           string(cloudenv.Get()),
 		NoLogsNoSupport: envknob.NoLogsNoSupport(),
 		AllowsUpdate:    envknob.AllowsRemoteUpdate(),
+		WoLMACs:         getWoLMACs(),
 	}
 }