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/ramdac/BT.c

@@ -35,7 +35,7 @@
 #include "BTPriv.h"
 #include "xf86RamDacPriv.h"
 
-_X_EXPORT void
+void
 BTramdacRestore(ScrnInfoPtr pScrn, RamDacRecPtr ramdacPtr,
 				    RamDacRegRecPtr ramdacReg)
 {
@@ -50,7 +50,7 @@ BTramdacRestore(ScrnInfoPtr pScrn, RamDacRecPtr ramdacPtr,
 						ramdacReg->DacRegs[i]);
 }
 
-_X_EXPORT void
+void
 BTramdacSave(ScrnInfoPtr pScrn, RamDacRecPtr ramdacPtr, 
 				 RamDacRegRecPtr ramdacReg)
 {
@@ -65,7 +65,7 @@ BTramdacSave(ScrnInfoPtr pScrn, RamDacRecPtr ramdacPtr,
 	    ramdacReg->DacRegs[i] = (*ramdacPtr->ReadDAC)(pScrn, i);
 }
 
-_X_EXPORT RamDacHelperRecPtr
+RamDacHelperRecPtr
 BTramdacProbe(ScrnInfoPtr pScrn, RamDacSupportedInfoRecPtr ramdacs/*, RamDacRecPtr ramdacPtr*/)
 {
     RamDacRecPtr ramdacPtr = RAMDACSCRPTR(pScrn);
@@ -137,7 +137,7 @@ BTramdacProbe(ScrnInfoPtr pScrn, RamDacSupportedInfoRecPtr ramdacs/*, RamDacRecP
     return ramdacHelperPtr;
 }
 
-_X_EXPORT void
+void
 BTramdacSetBpp(ScrnInfoPtr pScrn, RamDacRegRecPtr ramdacReg)
 {
     /* We need to deal with Direct Colour visuals for 8bpp and other