Refactor Haddock options

This patch renames the DOC_OPTIONS pragma to OPTIONS_HADDOCK. It also
adds "-- # ..."-style Haddock option pragmas, for compatibility with
code that use them.

Another change is that both of these two pragmas behave like
OPTIONS_GHC, i.e. they are only allowed at the top of the module, they
are ignored everywhere else and they are stored in the dynflags. There is
no longer any Haddock options in HsSyn.

Please merge this to the 6.8.2 branch when 6.8.1 is out, if appropriate.