[Xlibs] Janitorial works

[Luciano Montanaro]

I have been browsing through the X11 sources lately, thinking what kind
of stuff I could do without damage.

Two things came to me:
- the removal of the _Xconst macro - since the K&R prototypes are going,
  this should go too, or is there any different reason for it?
- doxygen documentation for the interface functions 
  (see http://www.doxygen.org). the documentation should already be 
  available for the man pages. It could be processed with some script,
  and inserted near the function definition.

Luciano