Better unavailability explanation after solver failure

using the code from OpamState.unavailable_reason
Pop an editor when pinning without an opam file
Still ask for confirmation for --fake actions

(they are actually more dangerous than normal ones...)
Merge pull request #1398 from OCamlPro/explain-unavailable-dep

Better unavailability explanation after solver failure
Merge pull request #1399 from OCamlPro/edit-no-opam

Allow pinning non-repository package without metadata
Fix: `OpamMisc.prettily_path` may raises `Not_found`
When stdin is /dev/null, the default answer of 'OpamState.confirm' is 'no'.

Close #1407
Update cmdliner to 0.9.4
Update URL to Gforge external components.
Merge pull request #1414 from rdicosmo/dose-trunk

Update URL to Gforge external components.
Merge pull request #1412 from chaudhuri/cmdliner_to_0.9.4

Update cmdliner to 0.9.4
Merge pull request #1409 from hnrgrgr/fix_end_of_file

When stdin is /dev/null, the default answer of 'OpamState.confirm' is 'no'
Closes #1407
Merge pull request #1408 from hnrgrgr/fix_getenv

Fix: `OpamMisc.prettily_path` may raises `Not_found`
Better printing of conflicts

some causes wouldn't be merged when they concerned the last step depending
on a non-existing package, eg:
opam install ackdo-reloaded.0.3 --switch 4.00.0 --show
Avoid a 'Sys_error' with 'opam pin'

When pinning to a local directory (or a git), if the source directory
contains files in 'opam/files', opam may raise

  Sys_error(.../opam/files/ocsigenserver.install: No such file or directory)'
Merge pull request #1421 from hnrgrgr/fix_not_found_pin_files

Avoid a 'Sys_error' with 'opam pin'
Merge pull request #1419 from OCamlPro/merge-conflicts

Better printing of conflicts
Small Makefile fixes
Merge pull request #1426 from OCamlPro/mf-fixes

Small Makefile fixes
Add 'opam config cudf-universe' to more reliably dump the whole cudf universe