[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/vma.def

@@ -22,12 +22,12 @@ object vma : object {
     }
 
     method map [cap:map] {
-        param process ref process
+        param process ref process [optional]
         param address address
     }
 
     method unmap [cap:unmap] {
-        param process ref process
+        param process ref process [optional]
     }
 
     method resize [cap:resize] {