ipn/ipnstate, wgengine/wgint: add handshake attempts accessors

Not yet used. This is being made available so magicsock/wgengine can
use it to ignore certain sends (UDP + DERP) later on at least mobile,
letting wireguard-go think it's doing its full attempt schedule, but
we can cut it short conditionally based on what we know from the
control plane.

Updates #7617

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: Ia367cf6bd87b2aeedd3c6f4989528acdb6773ca7

ipn/ipnstate/ipnstate.go

@@ -196,6 +196,10 @@ type PeerStatusLite struct {
 	LastHandshake time.Time
 	// NodeKey is this peer's public node key.
 	NodeKey key.NodePublic
+	// HandshakeAttempts is how many failed attempts there have been at
+	// completing the current WireGuard handshake. This resets to zero on every
+	// successful handshake.
+	HandshakeAttempts uint32
 }
 
 // PeerStatus describes a peer node and its current state.