Merge pull request #1527 from OCamlPro/fix-trim

Fixed small bug in the heuristic (targetting opam-rt)
Update CHANGES
Add an OPAMSKIPVERSIONCHECKS variable
Print a warning on constrained depopts for opam-version >= 1.2 packages

transparent at the moment and will help the migration
Make 'opam reinstall' call the solver

this may be needed:
- if the dependency changed in the meantime (repo or pin, more likely to happen now)
- when using eg. 'opam reinstall --build-tests' when there are test dependencies
Fix: advertise the 'install' field
Better pinning error message
Extend action cause printing: reinstalls can now trigger other actions
Fix reinstall for the internal solver
check_conflicts: mark packages with local metadata as available