[kernel] Pass objects not handles to syscall impls

This commit contains a couple large, interdependent changes:

- In preparation for capability checking, the _syscall_verify_*
  functions now load most handles passed in, and verify that they exist
  and are of the correct type. Lists and out-handles are not converted
  to objects.
- Also in preparation for capability checking, the internal
  representation of handles has changed. j6_handle_t is now 32 bits, and
  a new j6_cap_t (also 32 bits) is added. Handles of a process are now a
  util::map<j6_handle_t, handle> where handle is a new struct containing
  the id, capabilities, and object pointer.
- The kernel object definition DSL gained a few changes to support auto
  generating the handle -> object conversion in the _syscall_verify_*
  functions, mostly knowing the object type, and an optional "cname"
  attribute on objects where their names differ from C++ code.
  (Specifically vma/vm_area)
- Kernel object code and other code under kernel/objects is now in a new
  obj:: namespace, because fuck you <cstdlib> for putting "system" in
  the global namespace. Why even have that header then?
- Kernel object types constructed with the construct_handle helper now
  have a creation_caps static member to declare what capabilities a
  newly created object's handle should have.

src/kernel/objects/kobject.cpp

@@ -6,7 +6,8 @@
 #include "objects/kobject.h"
 #include "objects/thread.h"
 
-// TODO: per-cpu this?
+namespace obj {
+
 static j6_koid_t next_koids [static_cast<size_t>(kobject::type::max)] = { 0 };
 
 kobject::kobject(type t, j6_signal_t signals) :
@@ -26,7 +27,8 @@ kobject::koid_generate(type t)
 {
     kassert(t < type::max, "Object type out of bounds");
     uint64_t type_int = static_cast<uint64_t>(t);
-    return (type_int << 48) | next_koids[type_int]++;
+    uint64_t id = __atomic_fetch_add(&next_koids[type_int], 1, __ATOMIC_SEQ_CST);
+    return (type_int << 48) | id;
 }
 
 kobject::type
@@ -74,3 +76,5 @@ kobject::on_no_handles()
 {
     assert_signal(j6_signal_no_handles);
 }
+
+} // namespace obj