[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/vma.def

@@ -4,27 +4,33 @@ object vma : kobject {
     uid d6a12b63b3ed3937
     cname vm_area
 
+    capabilities [
+        map
+        unmap
+        resize
+    ]
+
     method create [constructor] {
         param size size
         param flags uint32
     }
 
-    method create_map [constructor] {
+    method create_map [constructor cap:map] {
         param size size
         param address address
         param flags uint32
     }
 
-    method map {
+    method map [cap:map] {
         param process object process
         param address address
     }
 
-    method unmap {
+    method unmap [cap:unmap] {
         param process object process
     }
 
-    method resize {
+    method resize [cap:resize] {
         param size size [inout]
     }
 }