[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/channel.cpp

@@ -5,7 +5,9 @@
 #include "objects/channel.h"
 #include "objects/vm_area.h"
 
-extern vm_area_guarded g_kernel_buffers;
+extern obj::vm_area_guarded g_kernel_buffers;
+
+namespace obj {
 
 constexpr size_t buffer_bytes = mem::kernel_buffer_pages * mem::frame_size;
 
@@ -89,3 +91,5 @@ channel::on_no_handles()
     kobject::on_no_handles();
     delete this;
 }
+
+} // namespace obj