ipn: split LocalBackend off into new ipn/ipnlocal package

And move a couple other types down into leafier packages.

Now cmd/tailscale doesn't bring in netlink, magicsock, wgengine, etc.

Fixes #1181

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

ipn/ipnstate/ipnstate.go

@@ -50,6 +50,12 @@ func (s *Status) Peers() []key.Public {
 	return kk
 }
 
+type PeerStatusLite struct {
+	TxBytes, RxBytes int64
+	LastHandshake    time.Time
+	NodeKey          tailcfg.NodeKey
+}
+
 type PeerStatus struct {
 	PublicKey key.Public
 	HostName  string // HostInfo's Hostname (not a DNS name or necessarily unique)