Differentiate between "global code" and "eval code" scripts.

Add a separate js_loadeval for "eval code" scripts, and let
js_do/loadstring create "global code" scripts.

js_newscript called with the NULL scope is equivalent to 'eval code'.
js_newscript called with the J->GE scape is equivalent to 'global code'.
js_newfunction is created with the lexical scope, i.e. 'function code'.