Make `unpin` an alias to `pin -u`

After all we have uninstall, so this makes sense and really is the most
intuitive. Closes #1250, thanks for the suggestions.
Cleanup the PIN command

This should address most issues raised by @dbuenzli in #1170.
`--force` has been removed, and OPAM will now automatically propose a
reinstall when appropriate.
Merge pull request #1256 from OCamlPro/pin-unpin

Cleanup the PIN command and add the `unpin` alias by popular demand