Uses/cabal.mk: Introduce CABAL_WRAPPER_SCRIPTS variable.

Before this change every Haskell executable was wrapped into a shell script
which was installed into ${PREFIX}/bin while the actual executable was installed
into {PREFIX}/libexec/cabal. This was required to set env variables pointing the
Haskell program to its data files under ${PREFIX}/share. However, not every
Haskell program uses this feature.

Now the shell wrapping is off by default and CABAL_WRAPPER_SCRIPTS knob can be
used to enable it for a given port/executable.

Adjust all Haskell ports affected by this change.