Add option '--safe' to guarantee absence of side-effects

Mostly useful for queries from scripts. For example, who would like TAB to trigger
a repository upgrade ??
Fixed 'opam repo list --short' to not display pinned packages
Add 'opam pin list --short'
Major refresh of bash_completion script

not very pretty, but a quick way to get a major UX improvment
Merge pull request #1695 from OCamlPro/better-completion

Major refresh of shell-completion. Should help browsing available options a lot.