[kernel] Add object_wait_many syscall

Add the object_wait_many syscall to allow programs to wait for signals
on multiple objects at once. Also removed the object argument to
thread::wait_on_signals, which does nothing with it. That information is
saved in the thread being in the object's blocked threads list.

src/kernel/syscalls/object.cpp

@@ -37,7 +37,7 @@ object_wait(j6_handle_t handle, j6_signal_t mask, j6_signal_t *sigs)
 
 	thread &th = thread::current();
 	obj->add_blocked_thread(&th);
-	th.wait_on_signals(obj, mask);
+	th.wait_on_signals(mask);
 
 	j6_status_t result = th.get_wait_result();
 	if (result == j6_status_ok) {
@@ -46,6 +46,55 @@ object_wait(j6_handle_t handle, j6_signal_t mask, j6_signal_t *sigs)
 	return result;
 }
 
+j6_status_t
+object_wait_many(j6_handle_t *handles, uint32_t count, j6_signal_t mask, j6_handle_t *handle, j6_signal_t *sigs)
+{
+    kutil::vector<kobject*> objects {count};
+
+    for (unsigned i = 0; i < count; ++i) {
+        j6_handle_t h = handles[i];
+        if (h == j6_handle_invalid)
+            continue;
+
+        kobject *obj = get_handle<kobject>(h);
+        if (!obj)
+            return j6_err_invalid_arg;
+
+        j6_signal_t current = obj->signals();
+        if ((current & mask) != 0) {
+            *sigs = current;
+            *handle = h;
+            return j6_status_ok;
+        }
+
+        objects.append(obj);
+    }
+
+	thread &th = thread::current();
+    for (auto *obj : objects)
+        obj->add_blocked_thread(&th);
+
+	th.wait_on_signals(mask);
+
+	j6_status_t result = th.get_wait_result();
+	if (result != j6_status_ok)
+        return result;
+
+    *handle = j6_handle_invalid;
+    *sigs = th.get_wait_data();
+    j6_koid_t koid = th.get_wait_object();
+    for (unsigned i = 0; i < count; ++i) {
+        if (koid == objects[i]->koid())
+            *handle = handles[i];
+        else
+            objects[i]->remove_blocked_thread(&th);
+    }
+
+    kassert(*handle != j6_handle_invalid,
+        "Somehow woke on a handle that was not waited on");
+    return j6_status_ok;
+}
+
 j6_status_t
 object_signal(j6_handle_t handle, j6_signal_t signals)
 {

src/kernel/syscalls/thread.cpp

@@ -40,7 +40,7 @@ j6_status_t
 thread_pause()
 {
 	thread &th = thread::current();
-	th.wait_on_signals(&th, -1ull);
+	th.wait_on_signals(-1ull);
 	return j6_status_ok;
 }