[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.

definitions/objects/thread.def

@@ -1,13 +1,17 @@
 object thread : kobject {
     uid 11f23e593d5761bd
 
+    capabilities [
+        kill
+    ]
+
     method create [constructor] {
-        param process object process
+        param process object process [cap:create_thread]
         param stack_top address
         param entrypoint address
     }
 
-    method kill [destructor]
+    method kill [destructor cap:kill]
 
     method exit [static] {
         param status int32