hostinfo: add SetDeviceModel setter, move remaining code from controlclient

Updates tailscale/corp#1959

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

control/controlclient/controlclient_test.go

@@ -75,10 +75,3 @@ func TestStatusEqual(t *testing.T) {
 		}
 	}
 }
-
-func TestOSVersion(t *testing.T) {
-	if osVersion == nil {
-		t.Skip("not available for OS")
-	}
-	t.Logf("Got: %#q", osVersion())
-}