Rework symbol visibility for easier maintenance

Save in a few special cases, _X_EXPORT should not be used in C source
files. Instead, it should be used in headers, and the proper C source
include that header. Some special cases are symbols that need to be
shared between modules, but not expected to be used by external drivers,
and symbols that are accessible via LoaderSymbol/dlopen.

  This patch also adds conditionally some new sdk header files, depending
on extensions enabled. These files were added to match pattern for
other extensions/modules, that is, have the headers "deciding" symbol
visibility in the sdk. These headers are:
o Xext/panoramiXsrv.h, Xext/panoramiX.h
o fbpict.h (unconditionally)
o vidmodeproc.h
o mioverlay.h (unconditionally, used only by xaa)
o xfixes.h (unconditionally, symbols required by dri2)

  LoaderSymbol and similar functions now don't have different prototypes,
in loaderProcs.h and xf86Module.h, so that both headers can be included,
without the need of defining IN_LOADER.

  xf86NewInputDevice() device prototype readded to xf86Xinput.h, but
not exported (and with a comment about it).

hw/xfree86/common/xf86Cursor.c

@@ -89,7 +89,7 @@ static Bool HardEdges;
  *      is valid if specified.
  */
 
-_X_EXPORT void
+void
 xf86InitViewport(ScrnInfoPtr pScr)
 {
 
@@ -133,7 +133,7 @@ xf86InitViewport(ScrnInfoPtr pScr)
  *      Scroll the visual part of the screen so the pointer is visible.
  */
 
-_X_EXPORT void
+void
 xf86SetViewport(ScreenPtr pScreen, int x, int y)
 {
   ScrnInfoPtr   pScr = XF86SCRNINFO(pScreen);
@@ -185,7 +185,7 @@ xf86PointerMoved(int scrnIndex, int x, int y)
  *	Enable/disable ZoomViewport
  */
 
-_X_EXPORT void
+void
 xf86LockZoom(ScreenPtr pScreen, Bool lock)
 {
   XF86SCRNINFO(pScreen)->zoomLocked = lock;
@@ -197,7 +197,7 @@ xf86LockZoom(ScreenPtr pScreen, Bool lock)
  *	set a new mode.
  */
 
-_X_EXPORT Bool
+Bool
 xf86SwitchMode(ScreenPtr pScreen, DisplayModePtr mode)
 {
   ScrnInfoPtr pScr = XF86SCRNINFO(pScreen);
@@ -322,7 +322,7 @@ xf86SwitchMode(ScreenPtr pScreen, DisplayModePtr mode)
  *      Reinitialize the visual part of the screen for another mode.
  */
 
-_X_EXPORT void
+void
 xf86ZoomViewport(ScreenPtr pScreen, int zoom)
 {
   ScrnInfoPtr    pScr = XF86SCRNINFO(pScreen);
@@ -480,7 +480,7 @@ xf86WarpCursor (DeviceIntPtr pDev, ScreenPtr pScreen, int x, int y)
 }
 
 
-_X_EXPORT void *
+void *
 xf86GetPointerScreenFuncs(void)
 {
     return (void *)&xf86PointerScreenFuncs;
@@ -579,7 +579,7 @@ FillOutEdge(xf86EdgePtr pEdge, int limit)
  * on 32 bit architectures, 64 on 64 bit architectures.
  */
 
-_X_EXPORT void
+void
 xf86InitOrigins(void)
 {
     unsigned long screensLeft, prevScreensLeft, mask;
@@ -845,7 +845,7 @@ xf86InitOrigins(void)
     }
 }
 
-_X_EXPORT void
+void
 xf86ReconfigureLayout(void)
 {
     int i;