Add support for a short description for each module, which is included
in the contents.

The short description should be given in a "Description: " field of
the header.  Included in this patch are changes that make the format
of the header a little more flexible.  From the comments:

-- all fields in the header are optional and have the form
--
-- [spaces1][field name][spaces] ":"
--    [text]"\n" ([spaces2][space][text]"\n" | [spaces]"\n")*
-- where each [spaces2] should have [spaces1] as a prefix.
--
-- Thus for the key "Description",
--
-- > Description : this is a
-- >    rather long
-- >
-- >    description
-- >
-- > The module comment starts here
--
-- the value will be "this is a .. description" and the rest will begin
-- at "The module comment".

The header fields must be in the following order: Module, Description,
Copyright, License, Maintainer, Stability, Portability.

Patches submitted by: George Russell <ger@informatik.uni-bremen.de>,
with a few small changes be me, mostly to merge with other recent
changes.

ToDo: document the module header.