Fix docstring for `haskell-menu-buffer-name`
Move inferior-haskell vars into new defgroup `inferior-haskell`

Next commit will do the same for the haskell-interactive-mode
variables. This makes it easier to keep apart the customization
variables for the two REPL modes.  This also adds an external info
manual link.
Move haskell-interactive vars into new defgroup `haskell-interactive`

This is a follow up to 602e373 which does the respective move for the
inf-haskell vars.  This makes it easier to keep apart the
customization variables for the two REPL modes.  This also adds an
external info manual link.
Add Info manual links to haskell{,-indent,indentation} defgroups
Add `:group` tags to derived modes

This way `M-x customize-mode` is guaranteed to show the appropriate
customization group.