[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/vm_area.h

@@ -14,6 +14,9 @@
 class page_tree;
 class vm_space;
 
+
+namespace obj {
+
 enum class vm_flags : uint32_t
 {
 #define VM_FLAG(name, v) name = v,
@@ -22,6 +25,7 @@ enum class vm_flags : uint32_t
     driver_mask     = 0x000fffff, ///< flags allowed via syscall for drivers
     user_mask       = 0x0000ffff, ///< flags allowed via syscall for non-drivers
 };
+is_bitfield(vm_flags);
 
 
 /// Virtual memory areas allow control over memory allocation
@@ -29,6 +33,9 @@ class vm_area :
     public kobject
 {
 public:
+    /// Capabilities on a newly constructed vma handle
+    constexpr static j6_cap_t creation_caps = 0;
+
     static constexpr kobject::type type = kobject::type::vma;
 
     /// Constructor.
@@ -170,5 +177,4 @@ private:
     uintptr_t m_next;
 };
 
-
-is_bitfield(vm_flags);
+} // namespace obj