runtime: introduce an atfork hook in the runtime

This commit introduce an atfork hook in the runtime, as a follow up to the issue encountered in MPR ocaml#455 and MPR ocaml#471.

To handle all the specific fork scenarios allowed in Multicore, a hook is required in order to let specific "alterations"
to the runtime do their job: by default, on a single domain program, only the domain lock needs to be reset.
When systhreads are in use, more cleaning up duty is required.

As a result, the usage of pthreads_atfork is also removed, opting to rely in both case on manually calling the
runtime hook when required.

otherlibs/systhreads/st_posix.h

@@ -396,7 +396,8 @@ int pthread_atfork(void (*prepare)(void), void (*parent)(void),
 
 static int st_atfork(void (*fn)(void))
 {
-  return pthread_atfork(NULL, NULL, fn);
+  caml_atfork_hook = fn;
+  return 0;
 }
 
 /* Signal handling */

otherlibs/unix/fork.c

@@ -32,6 +32,7 @@ CAMLprim value unix_fork(value unit)
   CAML_EV_FLUSH();
 
   ret = fork();
+  if (ret == 0) caml_atfork_hook();
   if (ret == -1) uerror("fork", Nothing);
 
   CAML_EVENTLOG_DO({

runtime/caml/domain.h

@@ -93,6 +93,8 @@ CAMLextern void caml_leave_blocking_section(void);
 CAMLextern void (*caml_enter_blocking_section_hook)(void);
 CAMLextern void (*caml_leave_blocking_section_hook)(void);
 
+CAMLextern void (*caml_atfork_hook)(void);
+
 CAMLextern void (*caml_domain_start_hook)(void);
 CAMLextern void (*caml_domain_stop_hook)(void);
 

runtime/domain.c

@@ -1048,6 +1048,14 @@ CAMLexport void caml_enter_blocking_section() {
   caml_enter_blocking_section_hook();
 }
 
+/* default handler for unix_fork, will be called by unix_fork. */
+static void caml_atfork_default(void) {
+  caml_reset_domain_lock();
+  caml_acquire_domain_lock();
+}
+
+CAMLexport void (*caml_atfork_hook)(void) = caml_atfork_default;
+
 void caml_print_stats () {
   struct gc_stats s;
 #if defined(COLLECT_STATS) && defined(NATIVE_CODE)