The main output path has exited with status 0 on EPIPE since #232, but
auxiliary outputs still used println!/print!/io::stdout().flush().unwrap()
and panicked (exit code 101) when the reader of a piped stdout went away
early, e.g. 'bat --completion bash | head -n0'.
Route them through the existing convention instead: write!/writeln! to
io::stdout() and '?' so that Error::Io(BrokenPipe) reaches
default_error_handler, which exits with status 0. This covers
--completion, --config-file, --diagnostic (bugreport's print() becomes
format() + writeln!), --generate-config-file, the 'cache' subcommand
progress messages, the OSC title sequence of --set-terminal-title, and
the 'built without feature' notices.
Closes#4034
