Sync dose patches with dose git

This cleans up the patching process: I now maintain a dose git branch to generate a cleaner stream of patches.

This allows to rebuild the branch from dose git using
```
git checkout 3.1.2
git am ../opam/src_ext/patches/dose/*.patch
```

The other way around, synching the patches on the opam repo from the 3.1.2-opam branch of dose can be done with:
```
cd opam; git rm -f src_ext/patches/dose/*
cd ../dose; git format-patch 3.1.2..3.1.2-opam -o ../opam/src_ext/patches/dose
cd ../opam; git add src_ext/patches/dose
```

--

Note that the patches on the upgrade bug (#1104) have been omitted for now as they cause problems.
Ensure that internal heuristics are correctly called if external solvers are interrupted.
Cleanup the int/ext invocations of the solver

and avoid cross-invocations of the external solver from OpamHeuristic that are bound to fail
Remove obsolete comment
Add some build instructions in the README
Merge pull request #1234 from OCamlPro/cleanup-solver-invocation

Cleanup the int/ext invocations of the solver
Fixed the installer script for system installs

Plus added a safeguard against ppl running it as root.
Merge pull request #1237 from OCamlPro/fix-installer-script-root

Fixed the installer script for system installs
Fixed bogus warning when using internal solver
Compute Cudf versions for available packages only

This avoids unneeded work and gives better conflict messages (we could previously
have a constraint referring to a not-available package, and the version would
then only be printed as '??'. Now it's mapped to available versions.
Print meaningful messages when a package is not available.

Closes #595
Merge pull request #1238 from OCamlPro/fix-conflicts-messages

Smaller fixes for conflict and error messages
Removed from the upgrade section of the request packages
with explicit equality constraints: they need to go in
the install part of the request.
Merge pull request #1239 from rdicosmo/fix-request-for-internal-solver

Fix subtle mistake in request generation used for internal heuristics
Now handle all constraints in upgrade request to the internal solver (dose patch)
Reorder patches
Finished reordering patches
Merge pull request #1240 from rdicosmo/fix-request-for-internal-solver

Now handle all constraints in upgrade request to the internal solver (do...
Remove dependency to camlp4

big queue of dose patches here, but it's simply a 'git format-patch'
from the dedicated branch on the dose repo. This is transitory.
Merge pull request #1244 from OCamlPro/no-camlp4

Remove dependency to camlp4