Fix handling of version files for HTTP backends
Improve the message when updating the backends

This should help fixing #620 (and maybe #661 ?)
Be more verbose on 'opam update'

This should fix #661
Fix typos in 'opam init'
Remove spams in debug logs
Improve 'opam init' by only downloading urls.txt and index.tar.gz once.

For an unknown reason, this was done (at least) once, thus slowing the
whole process. Luckily, because of the "nice" 'opam update'
refactoring (which has introduced too many regressions), it is now
trivial to build the global state when the repositories have already
been udpated and so 'opam init' is much nicer now!