Merge pull request #363 from tuong/master

Fix the number of jobs by default, see 419f416472d7a36fc2114a82096cbd0247ad0283 .
Improve debugging of solver inputs.

Now, we can use --dry-run to not install anything (ie. we just ask the solver) and --cudf=foo to generate foo-<n>.cudf containing the request for the n-th call to the (internal or external) solver.

This should fix #364
Merge branch 'master' of github.com:OCamlPro/opam
Fix 'opam config env'

This is a regression introduced in e65ff1bce9249eea3c858f8181ee0cc0fadb544d which causes to display the full environment when doing 'opam config env' instead of just the bits modified by OPAM.

This should fix #365
Fix CUDF preamble

This should fix #366
Continue 75349033ed382fdbc6a0cecc926f71df6734da86 (issue #365).

The unit-tests are passing again.
Continue 396f0b5b84e21cdf60cde926963ea9647e1a6e81

Now we display the root cause for every action proposed by the solver. This should improve the user experience quite a bit ...
Update CHANGES
Clean-ups in the tests Makefile
Distinguish between packages explicitely installed by the user and the rest.

This let us uninstall unecessary packages when one root package is uninstalled.

Also, try to give some explanations when deleting packages as well.
Improve the root cause analysis of OPAM actions.
Clean-ups: OPAM now compiles with -warning-error A -w +A-4-9
Init ~/.opam/<switch>/installed.user with the initial packages (it was working before, because is that file is empty, then OPAM copies the content of ~/.opam/<switch>/installed).
Initialize installed.user when switching and importing packages
Code refactoring: change installed.user -> installed.roots

No semantic changes
Update CHANGES
Add --installed and --short options to 'opam switch list'

The --installed option only display the currently installed compiler switches.
The --short option make OPAM display the list of available 'opam switch' target (ie. available compiler + installed switches) on one line, separated by spaces.

This should fix #371
Fix the documentation for --short (as it can be used with 'opam switch list' as well now)
Fix the non-termination of OPAM when sometimes we install an arbitrary package, see 006293e1f8bfb317dab59308cabe4363c6b67675 .