scripts global scope (as opposed to js.global which is the instance global
scope), and having exit() define exit_code in js.scope instead of js.global.

This also sets exit_code in js.scope to null when preparing to execute a new
script.  If a new script starts in the same scope as an old one, the old
exit_code value will be destroyed.

This should only impact scripts where js.global != js.scope (bbs.exec()ed,
and mailsrvr)