Move LockOSThread into init

cmd/dlv/main.go

@@ -25,12 +25,14 @@ type term struct {
 
 const historyFile string = ".dbg_history"
 
-func main() {
+func init() {
 	// We must ensure here that we are running on the same thread during
 	// the execution of dbg. This is due to the fact that ptrace(2) expects
 	// all commands after PTRACE_ATTACH to come from the same thread.
 	runtime.LockOSThread()
+}
 
+func main() {
 	var (
 		pid     int
 		run     bool