daemon: rely on $SIG{__WARN__} for error output

daemon: rely on $SIG{__WARN__} for error output

warn/carp usage is unavoidable given Perl itself and standard
libraries, so just rely on localized $SIG{__WARN__} from
60d262483a4d6ddf (daemon: use per-listener SIG{__WARN__} callbacks, 2022-08-08)
for all error reporting.

While we're in the area, make some of the error handling more
consistent between IMAP/NNTP/POP3.