Set subreaper true in default linux config

This sets the subreaper to true in the default linux config as the
common usecase is to not run containerd as pid 1.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

cmd/containerd/config_linux.go

@@ -11,6 +11,7 @@ func defaultConfig() *server.Config {
 		GRPC: server.GRPCConfig{
 			Address: server.DefaultAddress,
 		},
+		Subreaper: true,
 		Debug: server.Debug{
 			Level:   "info",
 			Address: server.DefaultDebugAddress,