[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.

src/kernel/objects/system.h

@@ -2,6 +2,7 @@
 /// \file system.h
 /// Definition of kobject type representing the system
 
+#include <j6/caps.h>
 #include "objects/kobject.h"
 
 namespace obj {
@@ -11,7 +12,7 @@ class system :
 {
 public:
     /// Capabilities on system given to init
-    constexpr static j6_cap_t init_caps = 0;
+    constexpr static j6_cap_t init_caps = j6_cap_system_all;
 
     static constexpr kobject::type type = kobject::type::system;