Included S-Lang updated to 1.4.5

[Pavel Roskin <proski gnu org>]

Hello!

I have updated the S-Lang library included with the mc sources to the 
version 1.4.5.

I actually wanted to fix a problem with the old version.  When selecting 
text in the viewer or in the editor using mouse on xterm, the text in the 
clipboard had spaces after the line ends.

Since Red Hat 8.0 packaged S-Lang with UTF-8 support, and there are many
problems with it, I wanted to have an option that I could recommend as a
temporary solution.  The problem with the clipboard would not allow me to
recommend the --with-screen=mcslang option to anyone.

Unfortunately, the changes required to be backported were too radical.  
To tell the terminal where the line ends, the new S-Lang uses a new
capability to erase text using current background.

So I decided to update S-Lang, and it was surprisingly easy.  To avoid 
renaming slang.h to slang-mc.h I moved all includes to a separate 
"include" directory.  This would prevent the configure script from picking 
local slang/slang.h header when testing for the installed library.

The new code has been tested on Red Hat 8.0 only.  Please give it more 
testing.

-- 
Regards,
Pavel Roskin