Fix 'opam config var list <pkg>'

Closes #1606
Fixed tput error (windows)

ref #246
Don't fail when listing available variables
Advertise additional compilers on 'opam switch list'

Closes #1575
Fix failure on some bad cl args

Closes #1644
Small package variables fix
Fix native & dynlink vars

Rename ocaml-tools-opt to ocaml-native-tools, and fix ocaml-native-dynlink for system
compilers (it now looks for `$(ocamlc -where)/dynlink.cmxa`, which should be more reliable.

Closes #1601, #1642
opam list: add option -A to list even unavailable packages, better doc

it could get misleading to pin a package and not find it with 'opam list', if uninstallable
Merge pull request #1666 from OCamlPro/small-fixes

A bunch of fixes on beta4, mostly UI