Returns true if 'arg' is an objectp and its file_name is securep() or
a file from /room/, /lib/ or /guild/. Especially /guild/ might change!
Don't expect that objects form /guild/ are systemp!
That's when an object is part of the mudlib and to be considered
'trustworthy' in security aspects.
If no argument is given 'previous_object()' is used as default.
