Save/restore js.scope property value in sbbs_t::js_execfile()

This solves the problem of exit() values (e.g. non-zero return codes) not
getting propagated to callers when nest-called (e.g. via bbs.exec()).

I think it was kk4qbn that pointed this out via IRC: an exit(1) call from
prextrn.js did not stop the external program from running (as it should, for
any non-zero exit code). This only happened when the prextrn.js called another
JS script (e.g. via bbs.exec() or as was the case here, indirectly via "EXEC"
@-code in the YesNoBar text.dat string (which executed yesnobar.js). This
nested JS script invocation via sbbs_t::js_execfile() would clobber the stored
js.scope property value (where the "exit_code" property is written).

Script invoked in their own context (e.g. via js.exec()) wouldn't have this
issue in the first place.