Add zephyr.downcase

Should be equivalent to zdowncase in the zephyr server. Could probably
implement this in JS, but I couldn't find an implementation of unicode
case-folding.
Truncate body components at NUL characters

Better to silently truncate (consistent with all of our other strings, sadly)
than to inject field separators. Issue #4.
Fix off-by-one error in notice body parsing

Message with a body of ['signature', ''] was being interpreted as
['signature']. This means that SERVACKs now look like ['SENT', ''], but it
seems they're supposed to look like that; they have a trailing NUL.