Use printf instead of echo -ne for xterm title

Fixes the problem noticed by Ariel Cornejo:

> Just a heads-up: BASH_COMMAND may contain escape sequences that you
> don't want "echo -ne" to parse. I learned this the hard way when
> smbclient -U yadda/yadda... '\\mymachine\c$' wouldn't show any output
> or prompts until it finished (\c means "suppress further output" which
> prevents the closing \007 from being printed).

-- https://mg.pov.lt/blog/bash-prompt.html#c1399262162.84