variadic C functions, fix XVaNestedList, sensible constant strings

* Enabled variadic C functions in Xlib.d (did not work previously due to const strings being char[] instead of char*)
* Changed constant strings to char* so they can be used with variadic functions
* Fixed XVaNestedList (should be type instead of pointer)
* Changed XInternAtom char* parameter to const so it doesn't complain when passing const strings (probably not the only function that needs this)

someone should poke Walter until he embeds C-inclusion in D