Merge pull request #379 from cago/master

Update completion script for zsh
Register all the printers in one place (eg. in OpamSystem)
Improve the output of opam-mk-error on error.

This should help debugging #378
Do not use spawn a shell to do redirect when creating index.tar.gz. Apparently it can fail on some system and tar simply accepts an output file as argument.

This should fix #378
Merge branch 'master' of github.com:OCamlPro/opam
Fix typo. This should really fix #378
Improvements to opam-mk-repo.

* Warn when skipping non-existant packages.
* Do nothing if the user has provided no valid packages.
* Be more robust when trying to create index.tar.gz or urls.txt in empty repositories (ie. do nothing in this case).
Add the build flags to 'opam import'
Import/export the root packages as well.
'opam remove' should behave nicely when using with --fake
Installed packages are always available.

This means that you can remove a repository and continue to use the packages normally as long as they are installed. Once uninstalled (because of upgrade/remove), then they will disapear.

This should fix #309
Refactoring, no semantic changes
Fix 'opam remove' solver request.

* Manually the packages depending on the ones we want to remove
* keep the same version for the transitve closure of soft-dependencies of installed roots
* and call the solver which will delete all the remaining non-root packages
'opam init' now accepts --fake (ie. to fake the installation of base packages)
When using the external solver, also display the command used to call the solver at the first line of the generated CUDF file (if using --cudf option)
Display a line on the CUDF file to say when we use the internal solver
Display a message when using --dry-run
By default, use the 'paranoid' strategy when an external solver is available.

If you want to tweak the stategy used, change OPAMCRITERIA (for instance OPAMCRITERIA=-new,-remove), see /usr/bin/aspcud for available options (the package is available under debian in apt-cudf.
Test. Do not pollute OPAM root with the export file.
Add a success message when the test is successful (sorry @altgr no color)