search: use "z:" instead of "bytes:" prefix

search: use "z:" instead of "bytes:" prefix

So far, searching by size has never been publicly documented,
and IMHO, of questionable utility.  In any case, "z:" is what
mairix(1) uses, so it may be familiar to existing mairix users
(I've never used this prefix myself).

So far, this prefix is only used internally in tests and in
auto-translated queries from IMAP; thus this incompatible change
is unlikely to affect anyone.