Fix --with-haddock option
It was broken in various ways :/
* The CmdHaddock (and CmdHaddockProject) have to restore the paths set with
'haddockProgramPath' (or 'haddockProjectProgramPath');
* haddock is not by default in 'ProgramDb'; We need to add it first,
otherwise 'reconfigure' will not update its path.
* 'setupHsHaddockFlags', need to store haddock location in
'haddockProgramPaths.
This patch also adds 'haddockProjectProgramPath' and
'haddockProjectProgramArgs' to 'HaddockProjectFlags' which are passed to
'CmdHaddockProject'.