Align the action list for better readability
Make --dry-run a real dry-run, and add --show to check the update

hmm on second thought maybe --show is not clear enough. Suggestions ?
Closes #1142
make opam list <pattern> list uninstalled packages
Ask for confirmation whenever there may be surprises

Previous behaviour was to do it unless there was a single change, which
could surprise for 'opam upgrade'.
New behaviour is to confirm when the impacted packages weren't exactly the ones
on your command-line.

Closes #1165
Merge pull request #1176 from OCamlPro/command-line-args

Various improvements to the command-line arguments