Handle program interruption while sandbox or local repo is enabled

[panglesd]

Various parts of ocaml-platform rely on temporarily modifying the opam state.

We should make sure that these temporary things are cleaned as much as possible.

As it might not be possible to clean them in all situation (eg a sudden shutdown of the computer), we should also clean them at the next run of the binary, and not rely on getting a clean state.

The temporary things are:

• The sandbox switch
• Enabling the repo

The program interuption that can happen are

• The user hit Ctrl-C. (I think this one is already fine if Sys.catch_break is true by default)
• The program receive a kill signal (we should handle it to turn it into an exception)
• The computer is shut down: nothing can be done, we should clean next run of the binary! Maybe using a different opam root leaves a better state for the user.

[panglesd]

The following was suggested during a dev meeting:

• Use a local switch in a temporary directory (to ensure eventual deletion)
• hide it from the user using a fancy opam command

[panglesd]

#69 and #67 deal with most of the case. To close this issue, we should also handle kill signals, if the user shut his computer down, and I think that would be a sufficient case coverage.