More tweaks to opam-admin findlib

the --install option will only try to install the packages if the findlib file
is empty. That will save lots of useless reinstallations.
Be more precise when computing the orphans packages
`opam-admin findlib --orphans` should print nothing if there is no orphans

(not even an empty line)
Merge pull request #1678 from samoht/master

More tweaks to opam-admin findlib