.install files, support for installing in etc.
Merge pull request #791 from dbuenzli/master

.install files, support for installing in etc.
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Merge branch 'master' of github.com:OCamlPro/opam
'opam config var pkg:pinned' should work even if the package is not installed

Fix #743
Fix the package stats computation

This is related to #793, although not fixing it completely.
[update] Do not fail when a pinned package is not installed
[update] do not display non-installed pinned packages as a DELETED package
Add more debug info
[pin] Remove the dev-package dir when a pinned package do not install properly

Fix #768
Better handling of package removed from upstream
Fixed 'opam install' when some packages have been removed from upstream
Fixed upgrade when packages have been removed from upstream
The solver needs to know about those packages to be able to remove them, which was not the case ;
there is still a glitch if you try eg to reinstall 'a' when 'b' depends on 'a': nothing will be
done about it, since 'b' can't be recompiled. If 'c' depends on 'b', it won't be recompiled either.

But since there is no clear way of handling that situation (the only correct thing would be to remove b and c ; that would need some more work),
it seems reasonable for now to ask for an 'opam upgrade' in such pathological cases.

Closes: #740