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).

randr/mirandr.c

@@ -34,7 +34,7 @@
 #include "randrstr.h"
 #include <stdio.h>
 
-_X_EXPORT Bool
+Bool
 miRRGetInfo (ScreenPtr pScreen, Rotation *rotations)
 {
     return TRUE;
@@ -44,7 +44,7 @@ miRRGetInfo (ScreenPtr pScreen, Rotation *rotations)
  * Any hardware that can actually change anything will need something
  * different here
  */
-_X_EXPORT Bool
+Bool
 miRRCrtcSet (ScreenPtr	pScreen,
 	     RRCrtcPtr	crtc,
 	     RRModePtr	mode,
@@ -64,7 +64,7 @@ miRRCrtcSetGamma (ScreenPtr	pScreen,
     return TRUE;
 }
 
-_X_EXPORT Bool
+Bool
 miRROutputSetProperty (ScreenPtr	    pScreen,
 		       RROutputPtr	    output,
 		       Atom		    property,
@@ -73,7 +73,7 @@ miRROutputSetProperty (ScreenPtr	    pScreen,
     return TRUE;
 }
 
-_X_EXPORT Bool
+Bool
 miRROutputGetProperty (ScreenPtr	    pScreen,
 		       RROutputPtr	    output,
 		       Atom		    property)
@@ -81,7 +81,7 @@ miRROutputGetProperty (ScreenPtr	    pScreen,
 	return TRUE;
 }
 
-_X_EXPORT Bool
+Bool
 miRROutputValidateMode (ScreenPtr	    pScreen,
 			RROutputPtr	    output,
 			RRModePtr	    mode)
@@ -89,7 +89,7 @@ miRROutputValidateMode (ScreenPtr	    pScreen,
     return FALSE;
 }
 
-_X_EXPORT void
+void
 miRRModeDestroy (ScreenPtr  pScreen,
 		 RRModePtr  mode)
 {
@@ -104,7 +104,7 @@ miRRModeDestroy (ScreenPtr  pScreen,
  * XXX what to do here....
  */
 
-_X_EXPORT Bool
+Bool
 miRandRInit (ScreenPtr pScreen)
 {
     rrScrPrivPtr    pScrPriv;