Rsync backend: allow remotes

this should allow all ssh remotes permitted by rsync. Still dubbed 'local' backend
in the code, and needs testing

use e.g. `user@host:dir/to/project/`. Don't forget the trailing slash.
Improve default preferences to get the latest versions for new dependencies
config report: some more solver info
Disabling disrupting 'cache of metadata' message
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.
Merge pull request #1677 from OCamlPro/solver-reports

Some more solver info on 'config report'
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 #1675 from rdicosmo/updatepreferences

Improve default preferences to get the latest versions for new dependencies
Merge pull request #1678 from samoht/master

More tweaks to opam-admin findlib
Forward-compatibility

This enables a nicer warning on files advertising a newer opam-version than the exe,
enabling for those "permissive mode" that will parse what is compatible and silently
ignore what it doesn't understand (__the whole field__; granularity could be improved)

Still fails with newer MAJOR version number, or if $OPAMROOT/config advertises a newer
version

ref: #1662
Ensure repo redirections are'nt broken by opam-version checks
'make fast' fix
Merge pull request #1680 from OCamlPro/forward-compat

Attempt to improve forward-compat
Fix SIGPIPE handling for sub-processes (thanks to @planar)
Be more explicit about the system ocaml version in `opam switch list`

Fixes #1685
Simple opam file checks and validation
Fix auto-detection of dev archives

If the only file in the directory is not a valid archive, consider this to be a dev package.
Merge pull request #1689 from samoht/master

Fix auto-detection of dev archives
Merge pull request #1686 from OCamlPro/fix-sigpipe

Fix SIGPIPE handling for sub-processes (thanks to @planar)