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).

include/privates.h

@@ -27,31 +27,31 @@ typedef struct _Private PrivateRec;
  * Request pre-allocated private space for your driver/module.
  * Calling this is not necessary if only a pointer by itself is needed.
  */
-extern int
+extern _X_EXPORT int
 dixRequestPrivate(const DevPrivateKey key, unsigned size);
 
 /*
  * Allocates a new private and attaches it to an existing object.
  */
-extern pointer *
+extern _X_EXPORT pointer *
 dixAllocatePrivate(PrivateRec **privates, const DevPrivateKey key);
 
 /*
  * Look up a private pointer.
  */
-pointer
+extern _X_EXPORT pointer
 dixLookupPrivate(PrivateRec **privates, const DevPrivateKey key);
 
 /*
  * Look up the address of a private pointer.
  */
-pointer *
+extern _X_EXPORT pointer *
 dixLookupPrivateAddr(PrivateRec **privates, const DevPrivateKey key);
 
 /*
  * Set a private pointer.
  */
-int
+extern _X_EXPORT int
 dixSetPrivate(PrivateRec **privates, const DevPrivateKey key, pointer val);
 
 /*
@@ -63,24 +63,24 @@ typedef struct _PrivateCallback {
     pointer *value;	/* address of private pointer */
 } PrivateCallbackRec;
 
-extern int
+extern _X_EXPORT int
 dixRegisterPrivateInitFunc(const DevPrivateKey key, 
 			   CallbackProcPtr callback, pointer userdata);
 
-extern int
+extern _X_EXPORT int
 dixRegisterPrivateDeleteFunc(const DevPrivateKey key,
 			     CallbackProcPtr callback, pointer userdata);
 
 /*
  * Frees private data.
  */
-extern void
+extern _X_EXPORT void
 dixFreePrivates(PrivateRec *privates);
 
 /*
  * Resets the subsystem, called from the main loop.
  */
-extern int
+extern _X_EXPORT int
 dixResetPrivates(void);
 
 /*
@@ -93,14 +93,14 @@ dixResetPrivates(void);
  * Looks up the offset where the devPrivates field is located.
  * Returns -1 if no offset has been registered for the resource type.
  */
-extern int
+extern _X_EXPORT int
 dixLookupPrivateOffset(RESTYPE type);
 
 /*
  * Specifies the offset where the devPrivates field is located.
  * A negative value indicates no devPrivates field is available.
  */
-extern int
+extern _X_EXPORT int
 dixRegisterPrivateOffset(RESTYPE type, int offset);
 
 /*