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>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f