using the SLang library politely

[Roland Illig]

Hi,

I think we should replace the calls to _SLsys_getkey() and 
_SLsys_input_pending with calls to SLang_getkey() and 
SLang_input_pending. (see src/slint.c.)

The latter functions are from the public interface of SLang, the former 
ones are internal system-dependent functions. The SLang_* functions are 
wrappers around the _SLsys_* functions.

As we are _using_ the SLang library we should stick to the public 
interface. The functions starting with an underscore are -- as usual -- 
internal to the package (and used for _implementing_ it), so we should 
treat it as such.

Roland