More info at unpinning

Ref #1303
Allow pinning dev-packages without a repo

Closes #941

Would be good to make specification of package name/version from the metadata
work directly (name conflict will fail with a message at the moment)
Allow installing absent package on "opam reinstall"

(after a confirmation)
Closes #1082
Better error message when .install fails
Fix 'metadata changed' on new pins