[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/syscalls.def

@@ -1,9 +1,11 @@
-import "objects/system.def"
 import "objects/kobject.def"
-import "objects/process.def"
-import "objects/thread.def"
+
 import "objects/channel.def"
 import "objects/endpoint.def"
+import "objects/event.def"
+import "objects/process.def"
+import "objects/system.def"
+import "objects/thread.def"
 import "objects/vma.def"
 
 interface syscalls [syscall] {
@@ -11,6 +13,7 @@ interface syscalls [syscall] {
 
     expose object system
     expose object kobject
+    expose object event
     expose object process
     expose object thread
     expose object channel
@@ -24,4 +27,11 @@ interface syscalls [syscall] {
     function log {
         param message string
     }
+
+    # Get a list of handles owned by this process. If the
+    # supplied list is not big enough, will set the size
+    # needed in `size` and return j6_err_insufficient
+    function handle_list {
+        param handles object kobject [list inout optional] # A list of handles to be filled
+    }
 }