[kernel] Add handle_clone syscall

Added the handle_clone syscall which allows for cloning a handle with
a subset of the original handle's capabilities.

Related changes:

- srv.init now calls handle_clone on its system handle, and load_program
  was changed to allow this second system handle to be passed to loaded
  programs instead. However, as drv.uart is still a driver AND a log
  reader, this new handle is not actually passed yet.
- The definition parser was using a set for the cap list, which meant
  the order (and thus values) of caps was not static.
- Some code in objects/handle.h was made more explicit about what bits
  meant what.

definitions/syscalls.def

@@ -34,4 +34,12 @@ interface syscalls [syscall] {
     function handle_list {
         param handles object kobject [list inout optional] # A list of handles to be filled
     }
+
+    # Create a clone of an existing handle, possibly with
+    # some capabilities masked out.
+    function handle_clone {
+        param handle object kobject [handle] # The handle to clone
+        param clone object kobject [out]     # The new handle
+        param mask uint32                    # The capability bitmask
+    }
 }