[kernel] Remove process & thread self-handles

For the coming switch to cap/handle ref-counting being the main lifetime
determiner of objects, get rid of self handles for threads and processes
to avoid circular references. Instead, passing 0 to syscalls expecting a
thread or process handle signifies "this process/thread".

definitions/objects/thread.def

@@ -7,7 +7,7 @@ object thread : object {
     ]
 
     method create [constructor] {
-        param process ref process [cap:create_thread]
+        param process ref process [optional cap:create_thread]
         param stack_top address
         param entrypoint address
         param arg0 uint64