[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/errors.py

@@ -1,2 +1,3 @@
 class InvalidType(Exception): pass
 class UnknownTypeError(Exception): pass
+class UnknownCapError(Exception): pass