Rename our global Lua object.

This avoids the warning on all the primitive functions:

    warning: declaration shadows a local variable [-Wshadow]
Fixed the final use - to avoid compilation error.
  Added missing prototypes.
  Avoid shadowing the "screen" variable.
  Misc fixups.
  Explicitly cast the return of `luaL_checknumber` to int.

  This avoids an implicit conversion
  Avoid unreachable code - break after exit().

  Changed the exit to return which will do the same job.
 Misc cleanup.

  Avoid shadowing the screen-variable.
  Avoid unreachable code.
  Avoid warnings about missing prototypes.

  Interestingly this raises a warning:

        void invoke_lua();

  But this does not:

        void invoke_lua(void);
  Avoid unreachable code - remove "break" label
Merge pull request #5 from skx/clang

Allow compilation free of warnings from clang.