Big patch for mcview

[Roland Illig <roland illig gmx de>]

Hi all,

I must have had too much time, as I have partly rewritten mcview. The
patch is quite large and changes many things. However, I have tested it
with all available types of data sources (see the code for explanation),
and it seems to work.

http://www.roland-illig.de/tmp/viewer.patch

Here are the major improvements:

* Dropped mmap support.

* Don't load large files completely into memory.

* Grouped all variables that have to do with the data source management.

* Made every variable have only one purpose. (Before, you could never be
   sure what the view->data field contained, as it was used for keeping
   the error message as well as the mmapped file and the data of the
   non-mmapped file.)

* Provided a clean interface for the get_byte() function, as well as
   four different implementations of it.

* Added assertions to guarantee that the code is only used in situations
   where it is meant to be used.

* First steps to supporting 64-bit files. (Mainly through consistent
   use of the appropriate data types.)

Perhaps you may worry about the fact that my patch makes the file 119
lines longer, but I hope the code becomes more readable and maintainable.

Of course, there are many things that don't work yet or that can be
improved. I will work on them, perhaps by first marking the relevant
places with FIXMEs. Things that don't work nicely at the moment are:

* Initialization of a WView object. (I'd like to have a function that
   initializes _every_ field to a sensible default value; "constructor".)

* Destruction of a WView object. (The counterpart to the constructor.
   Cleans up the object and frees it.)

* Cursor movement in all display modes (textview, textwrap, hexview,
   hexedit, rawmode, nroffmode, ...) should be rewritten in cleaner code.

* Support for 64-bit files still needs much work.

Roland