[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/thread.cpp

@@ -10,9 +10,12 @@
 #include "scheduler.h"
 
 extern "C" void kernel_to_user_trampoline();
-static constexpr j6_signal_t thread_default_signals = 0;
+extern obj::vm_area_guarded &g_kernel_stacks;
 
-extern vm_area_guarded &g_kernel_stacks;
+
+namespace obj {
+
+static constexpr j6_signal_t thread_default_signals = 0;
 
 thread::thread(process &parent, uint8_t pri, uintptr_t rsp0) :
     kobject(kobject::type::thread, thread_default_signals),
@@ -32,7 +35,7 @@ thread::thread(process &parent, uint8_t pri, uintptr_t rsp0) :
         m_tcb.rsp0 = rsp0;
 
     m_creator = current_cpu().thread;
-    m_self_handle = parent.add_handle(this);
+    m_self_handle = parent.add_handle(this, thread::parent_caps);
 }
 
 thread::~thread()
@@ -246,3 +249,5 @@ thread::create_idle_thread(process &kernel, uint8_t pri, uintptr_t rsp0)
     idle->set_state(state::ready);
     return idle;
 }
+
+} // namespace obj