Use http.debian.net as default Debian mirror if no mirror is given

... i.e. if even xt-guess-suite-and-mirror is not used.
Also use http.debian.net as default mirror for the debian-archive
Document 4.3 release date in NEWS
Rename NEWS to NEWS.markdown
Also switch xt-guess-suite-and-mirror's fallback to http.debian.net
Document 4.3.1 in NEWS
Use Test::NoTabs in t/no-tabs.t