More fixes for older GHC versions.

These changesets, originally based on benarmstron's fork, provide fixes for the issues with command line flags for older ghc versions you mentioned [in the previous pull request](https://github.com/tmhedberg/hsenv/pull/3). I tested the changes with GHC 7.0.4 (Fedora's package), 7.4.2 and 7.6.1 (private hsenv installs). The biggest change is that the activate script now exports different package-db option strings for ghc, ghc-pkg and cabal, adjusting them according to the GHC version (thus handling the change from -package-conf to -package-db in 7.6.1). Smaller changes:

- If GHC is being installed from a tarball, symlinks will be created to any tools which exist in the newly-created private installation of GHC.

- The Control.Exception import fixes in your master branch are already included.