control/controlclient, tailcfg: add Node.Expired field, set for expired nodes

Nodes that are expired, taking into account the time delta calculated
from MapResponse.ControlTime have the newly-added Expired boolean set.
For additional defense-in-depth, also replicate what control does and
clear the Endpoints and DERP fields, and additionally set the node key
to a bogus value.

Updates #6932

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ia2bd6b56064416feee28aef5699ca7090940662a

tailcfg/tailcfg_view.go

@@ -175,6 +175,7 @@ func (v NodeView) UnsignedPeerAPIOnly() bool         { return v.ж.UnsignedPeerA
 func (v NodeView) ComputedName() string              { return v.ж.ComputedName }
 func (v NodeView) ComputedNameWithHost() string      { return v.ж.ComputedNameWithHost }
 func (v NodeView) DataPlaneAuditLogID() string       { return v.ж.DataPlaneAuditLogID }
+func (v NodeView) Expired() bool                     { return v.ж.Expired }
 func (v NodeView) Equal(v2 NodeView) bool            { return v.ж.Equal(v2.ж) }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -207,6 +208,7 @@ var _NodeViewNeedsRegeneration = Node(struct {
 	computedHostIfDifferent string
 	ComputedNameWithHost    string
 	DataPlaneAuditLogID     string
+	Expired                 bool
 }{})
 
 // View returns a readonly view of Hostinfo.