Stop leaking WWW/PSGI-specific logic into classes like
PublicInbox::Inbox, which is used universally.
We'll also decouple $ibx->over from $ibx->search and just deal
with duplicate the code inside ->over to reduce argument
complexity in ->search.
This is also a step in moving away from using {psgi.errors}
to ease code sharing between IMAP, NNTP, and command-line
interfaces. Perl's built-in `warn' and `local $SIG{__WARN__}'
provides all the flexibility we need to control warning output
and should be universally understood by Perl hackers who may
be unfamiliar with PSGI.