2.015.3: more general plumbing to prevent infinite recursion in defining systems.

A new exported macro with-system-definitions protects
operate (and through it oos, load-system and compile-system)
and find-system (and also its helper load-sysdef should it ever be exported)
and will hopefully complete the find-system-if-being-defined from 2.015.2
in preventing infinite recursions while defining systems that are defined
in a place different from the place where they are registered
(maybe there should be a warning for that?)