[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.

scripts/definitions/context.py

@@ -68,6 +68,14 @@ class Context:
         from .types import ObjectRef
         ObjectRef.connect(objects)
 
+        for obj in objects.values():
+            for method in obj.methods:
+                caps = method.options.get("cap", list())
+                for cap in caps:
+                    if not cap in obj.caps:
+                        from .errors import UnknownCapError
+                        raise UnknownCapError(f"Unknown capability {cap} on {obj.name}::{method.name}")
+
         self.objects.update(objects)
         self.interfaces.update(interfaces)