`struct object_rw` is poisoned prior to initializing and sending it.
However, some fields are legitimately unininitialized (other than their
memset or poison value) at send time. Add and use a poison variant that
can clear those fields, without marking them unreadable.
Headers included by SConstruct tests cannot rely on the existence of
"dxxsconf.h"; instead, SConstruct must define the symbols that would
have been in it.
