Don't initialise cabal.sandbox.config from ~/.cabal/config.

Reverts fada98e2d0d2fdda36476befdf4e9176f3e40dc7.
Refactoring.

Remove the confusing 'basePkgEnv' function, make loading the inherited package
environment more explicit.
Rename 'userPkgEnv' to 'userPackageEnvironment'.
Make 'createPackageEnvironment' return '()'.

Removes a bit of duplication.
Cosmetic change.
Makes sandboxes inherit from '~/.cabal/config'.

Previously that file was copied at sandbox creation time. Fixes #1123.
Small code reorg.

No functionality changes.
Don't show an uninformative message by default.
Merge pull request #1217 from 23Skidoo/sandbox-config

Make sandbox configs inherit from '~/.cabal/config'.