[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.

assets/grammars/definitions.g

@@ -2,7 +2,7 @@ start: import_statement* (object|interface)+
 
 import_statement: "import" PATH
 
-object: description? "object" name options? super? "{" uid cname? method* "}"
+object: description? "object" name options? super? "{" uid cname? capabilities? method* "}"
 
 interface: description? "interface" name options? "{" uid interface_param* "}"
 
@@ -14,6 +14,8 @@ uid: "uid" UID
 
 cname: "cname" IDENTIFIER
 
+capabilities: "capabilities" "[" IDENTIFIER+ "]"
+
 super: ":" name
 
 function: description? "function" name options? ("{" param* "}")?