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
keep opam's bin/ at the same place in the PATH list during a switch
Merge pull request #1189 from vprevosto/master

keep opam's bin/ at the same place in the PATH list during a switch
Make the argument of 'opam show' mandatory

if you _really_ want the info of all packages, use '*'
Don't display empty mirrors list

closes #1128
Fix another small case for action causes
Skip unneeded package downloads
Merge pull request #1195 from OCamlPro/small-UI-fixes

small display and command-line fixes
Merge pull request #1197 from OCamlPro/fix-unneeded-download

Skip unneeded package downloads
Changed default preferences to avoid downgrades when possible.