[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/channel.def

@@ -1,14 +1,20 @@
 object channel : kobject {
     uid 3ea38b96aa0e54c8
 
-    method create [constructor] 
-    method close [destructor]
+    capabilities [
+        send
+        receive
+        close
+    ]
 
-    method send {
+    method create [constructor] 
+    method close [destructor cap:close]
+
+    method send [cap:send] {
         param data buffer [inout]
     }
 
-    method receive {
+    method receive [cap:receive] {
         param data buffer [out]
     }
 }