Fix #651

OPAM now tries to fix dandling symlink for OPAM files on 'opam update', and for compiler files on each opam invocation (that's a bit too much maybe, but well ...)
Fix embarassing #648

This is a regression introduced by the 'quicker update' branch merge.
Remove spamm on 'opam init' introduced by last commit

Fix #663
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!
Change the default behavior of 'opam list' to use the same conventions as ocamlfind (ie. to display only the installed packages).

Use 'opam list -a' to list all the packages which are installable.

I'm not totally convinced by this change (actually I don't really care), but let see if people start screaming or not.

Fix #644
Actually, really change the behavior of 'opam install' (#644)
Update CHANGES
Speed-up base invocations of `opam config var <VAR>`

Now `opam config var prefix` is immediate, and ocp-index can use it nicely.
Update CHANGES
More messages when using --fake
Improve the display of directory names on update
Cosmetic changes in opamHeuristic

(debug messages + doc + better variable names)
Try to improve the messages display when uninstalling a package, to have hints of what could be the cause of #642
Fix tests.

Because of 30ff64067ef23a2bc76eed26f336a9355a29bae7, `opam list` can now be empty.
Continue #634