Fixed some exit codes

'opam list' returns 1 if no packages were found, 'opam install --show' and user-aborted installs return 0
Tiny Makefile fixes
Small UI fixes
Self-document the list of variables.

Also fixes obsolete --help text
Fixed lookup for composite variables (pkg1+pkg2:enable)
Merge pull request #1471 from OCamlPro/some-ui-fixes

Some UI fixes: document "opam config list", fix some return values...