[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/syscalls/handle.cpp

@@ -0,0 +1,27 @@
+#include <j6/errors.h>
+#include <j6/types.h>
+
+#include "objects/process.h"
+
+using namespace obj;
+
+namespace syscalls {
+
+j6_status_t
+handle_list(j6_handle_t *handles, size_t *handles_len)
+{
+    if (!handles_len || (*handles_len && !handles))
+        return j6_err_invalid_arg;
+
+    process &p = process::current();
+    size_t requested = *handles_len;
+
+    *handles_len = p.list_handles(handles, requested);
+
+    if (*handles_len < requested)
+        return j6_err_insufficient;
+
+    return j6_status_ok;
+}
+
+} // namespace syscalls