Implement shell-style boolean values for boolean env vars

Should be 0, no or false for disabled ; 1, yes or true for enabled
Any other value triggers a warning (or do we want to blindly assume "disabled" is yes ?).

Closes #1608
dl messages: repos in blue, packages in green

Closes #1621
Fix META.in for referencing dose3
Fix ext solver config for opam-rt
Merge pull request #1634 from OCamlPro/vars-and-disp

Small 1.2 UI fixes