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/xf86sbusBus.c

@@ -41,7 +41,7 @@
 #include "xf86sbusBus.h"
 #include "xf86Sbus.h"
 
-_X_EXPORT Bool sbusSlotClaimed = FALSE;
+Bool sbusSlotClaimed = FALSE;
 
 static int xf86nSbusInfo;
 
@@ -79,7 +79,7 @@ CheckSbusDevice(const char *device, int fbNum)
     psdp->fd = -1;
 }
 
-_X_EXPORT void
+void
 xf86SbusProbe(void)
 {
     int i, useProm = 0;
@@ -360,7 +360,7 @@ xf86ClaimSbusSlot(sbusDevicePtr psdp, DriverPtr drvp,
 	return -1;
 }
 
-_X_EXPORT int
+int
 xf86MatchSbusInstances(const char *driverName, int sbusDevId, 
 		       GDevPtr *devList, int numDevs, DriverPtr drvp,
 		       int **foundEntities)
@@ -536,7 +536,7 @@ xf86MatchSbusInstances(const char *driverName, int sbusDevId,
 /*
  * xf86GetSbusInfoForEntity() -- Get the sbusDevicePtr of entity.
  */
-_X_EXPORT sbusDevicePtr
+sbusDevicePtr
 xf86GetSbusInfoForEntity(int entityIndex)
 {
     sbusDevicePtr *psdpp;
@@ -552,7 +552,7 @@ xf86GetSbusInfoForEntity(int entityIndex)
     return NULL;
 }
 
-_X_EXPORT int
+int
 xf86GetEntityForSbusInfo(sbusDevicePtr psdp)
 {
     int i;
@@ -567,7 +567,7 @@ xf86GetEntityForSbusInfo(sbusDevicePtr psdp)
     return -1;
 }
 
-_X_EXPORT void
+void
 xf86SbusUseBuiltinMode(ScrnInfoPtr pScrn, sbusDevicePtr psdp)
 {
     DisplayModePtr mode;
@@ -667,7 +667,7 @@ xf86SbusCmapCloseScreen(int i, ScreenPtr pScreen)
     return (*pScreen->CloseScreen) (i, pScreen);
 }    
 
-_X_EXPORT Bool
+Bool
 xf86SbusHandleColormaps(ScreenPtr pScreen, sbusDevicePtr psdp)
 {
     sbusCmapPtr cmap;