Create symlinks to extra tools in private ghc/bin, if they exist.

It is a bit excessive to look for some of the executables (e.g. uhc) in a
ghc/bin directory, but I guess that is not a big deal.
Use -package-conf instead of -package-db in wrappers for ghc* < 7.6.1.

The -no-user-package-db flags were provisionally removed.
Fix an incorrect hardcoded path in the activate script
Adding back the -no-user-package-(db|conf) flags to the ghc and ghc-pkg options.

I suspect these flags have no effect, as the -package-(db|conf) options override
them. I might be wrong, however.
Improve skeleton compatibility with GHC 7.4 and any GHC installed from a tarball

This is a cleaned up version of the `multiple-ghc-compat` branch of
https://github.com/duplode/hsenv.git.

Merge branch 'multi_ghc_compat'