--with-screen implemented

[Pavel Roskin <proski gnu org>]

Hello!

I have committed the patch that uses one option --with-screen in place of 
3 options:

--with-slang          => --with-screen=slang
--with-included-slang => --with-screen=mcslang
--with-ncurses        => --with-screen=ncurses

Having one option instead of three makes it easier to avoid invalid
combinations of the options.  It should be obvious to everybody that a
choice needs to be made, and that S-Lang and ncusres cannot be supported
in the same time.

--with-termcap is now valid only if --with-screen=mcslang was specified or
if the included S-Lang was selected automatically.  There are no side
effects from specifying --with-termcap (i.e. it doesn't force using the
included S-Lang).

As a positive side effect of the rewriting, mc on FreeBSD 4.6.2 with
included S-Lang now used terminfo database by default (it was using
termcap).

To specify non-standard paths for includes and libraries, variables 
CPPFLAGS and LDFLAGS should be used, as documented in the output of 
"configure --help".

Files INSTALL and INSTALL.FAST have been updated to reflect the changes.

-- 
Regards,
Pavel Roskin