Stop cleanly if a program being debugged spawns a domain

Otherwise the debugger gets all confused.

It is still possible to debug the program up to the point it spawns a
new domain.  This is the same solution we used in ocaml#10594 for programs
that create threads.

runtime/domain.c

@@ -25,6 +25,7 @@
 #include "caml/alloc.h"
 #include "caml/backtrace.h"
 #include "caml/callback.h"
+#include "caml/debugger.h"
 #include "caml/domain.h"
 #include "caml/domain_state.h"
 #include "caml/runtime_events.h"
@@ -1112,6 +1113,10 @@ CAMLprim value caml_domain_spawn(value callback, value mutex)
   pthread_t th;
   int err;
 
+#ifndef NATIVE_CODE
+  if (caml_debugger_in_use)
+    caml_fatal_error("ocamldebug does not support spawning multiple domains");
+#endif
   p.parent = &domain_self->interruptor;
   p.status = Dom_starting;