Actions display: fixed bogus 'conflicts with' for non-existing packages
Fix printing of backtraces within switch_backup
Always look in the environment for user-provided variable overrides

This let us do:

```
export OPAMVAR_bin=/usr/local/bin && opam install foo
```
Fixed lack of confirmation when doing 'opam remove -a'
When applying a solution, download all data before proceeding
Better action processing, error recovery and user report
Disable 'error recovery'

that worked really unreliably, and actually made the errors much more confusing.
Since we provide switch backups in case of error, the 'switch import' command provides
the same functionality in a user-controlled, clear and more reliable manner
Correct handling of sigint in sub-processes
Merge pull request #1153 from samoht/master

Always look in the environment for user-provided variable overides
Merge pull request #1187 from OCamlPro/three-small-issues

Fixes small UI issues (lack of confirmation, backtrace lost, bogus action cause)
Merge remote-tracking branch 'OCamlPro/fix-trains'

Conflicts:
	src/client/opamArg.ml
Changed default preferences to avoid downgrades when possible.
Quote special shell chars in default OPAMCRITERIA
Proper quoting of user preferences possibly containing ( and ) now moved
to dose patch; added help entry for OPAMCRITERIA with default value printed out.
Revert useless quotation
Adding debug on external solver call
Debug, again