updated the docs and rebuild releasing 2.6.21 removed

* NEWS elfgcchack.h testapi.c doc/*: updated the docs and rebuild
  releasing 2.6.21
* include/libxml/threads.h threads.c: removed xmlIsThreadsEnabled()
* threads.c include/libxml/threads.h xmllint.c: added the more
  generic xmlHasFeature() as suggested by Bjorn Reese, xmllint uses it.
Daniel

threads.c

@@ -164,24 +164,6 @@ static xmlRMutexPtr	xmlLibraryLock = NULL;
 static void xmlOnceInit(void);
 #endif
 
-/**
- * xmlIsThreadsEnabled:
- *
- * Run-time information about whether multithread support is compiled in
- *
- * Returns 1 (true) if library was compiled with multithread support
- * enabled or 0 (false) otherwise.
- */
-int
-xmlIsThreadsEnabled(void)
-{
-#ifdef LIBXML_THREAD_ENABLED
-    return 1;
-#else
-    return 0;
-#endif
-}
-
 /**
  * xmlNewMutex:
  *