Always store child pid

Keep the mapping of all living child processes so that we'll be able
to iterate over them to clean up, rather than only storing the
children which have an associated port.

erts/emulator/sys/unix/erl_child_setup.c

@@ -644,13 +644,10 @@ static Hash *forker_hash;
 
 static void add_os_pid_to_port_id_mapping(Eterm port_id, pid_t os_pid)
 {
-    if (port_id != THE_NON_VALUE) {
-        /* exit status report requested */
-        ErtsSysExitStatus es;
-        es.os_pid = os_pid;
-        es.port_id = port_id;
-        hash_put(forker_hash, &es);
-    }
+    ErtsSysExitStatus es;
+    es.os_pid = os_pid;
+    es.port_id = port_id;
+    hash_put(forker_hash, &es);
 }
 
 static Eterm get_port_id(pid_t os_pid)