tailcfg: add some missing json omitempty

Noticed these in MapResponses to clients.

MachineAuthorized was set true, but once we fix the coordination server
to zero out that field, then it can be omittted.

tailcfg/tailcfg.go

@@ -151,9 +151,9 @@ type Node struct {
 	Created    time.Time
 	LastSeen   *time.Time `json:",omitempty"`
 
-	KeepAlive bool // open and keep open a connection to this peer
+	KeepAlive bool `json:",omitempty"` // open and keep open a connection to this peer
 
-	MachineAuthorized bool // TODO(crawshaw): replace with MachineStatus
+	MachineAuthorized bool `json:",omitempty"` // TODO(crawshaw): replace with MachineStatus
 }
 
 type MachineStatus int
@@ -279,7 +279,7 @@ type Service struct {
 type Hostinfo struct {
 	// TODO(crawshaw): mark all these fields ",omitempty" when all the
 	// iOS apps are updated with the latest swift version of this struct.
-	IPNVersion    string       // version of this code
+	IPNVersion    string       `json:",omitempty"` // version of this code
 	FrontendLogID string       `json:",omitempty"` // logtail ID of frontend instance
 	BackendLogID  string       `json:",omitempty"` // logtail ID of backend instance
 	OS            string       // operating system the client runs on (a version.OS value)