Choose solver criteria depending on the action

This resolves the problem of unrelated packages installed on 'opam install'
by minimising the changes except on 'opam upgrade'. You may now specify
OPAMUPGRADECRITERIA. OPAMCRITERIA and --criteria still change the criteria
for all actions.
Merge pull request #1364 from OCamlPro/upgrade-criteria

Choose solver criteria depending on the action