[kernel] Add capabilities to handles

This change finally adds capabilities to handles. Included changes:

- j6_handle_t is now again 64 bits, with the highest 8 bits being a type
  code, and the next highest 24 bits being the capability mask, so that
  programs can check type/caps without calling the kernel.
- The definitions grammar now includes a `capabilities [ ]` section on
  objects, to list what capabilities are relevant.
- j6/caps.h is auto-generated from object capability lists
- init_libj6 again sets __handle_self and __handle_sys, this is a bit
  of a hack.
- A new syscall, j6_handle_list, will return the list of existing
  handles owned by the calling process.
- syscall_verify.cpp.cog now actually checks that the needed
  capabilities exist on handles before allowing the call.

src/kernel/objects/vm_area.h

@@ -5,6 +5,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <j6/caps.h>
 #include <j6/signals.h>
 #include <util/vector.h>
 #include <util/enum_bitfields.h>
@@ -34,7 +35,7 @@ class vm_area :
 {
 public:
     /// Capabilities on a newly constructed vma handle
-    constexpr static j6_cap_t creation_caps = 0;
+    constexpr static j6_cap_t creation_caps = j6_cap_vma_all;
 
     static constexpr kobject::type type = kobject::type::vma;