Keep track of the windows and hooks used by a thread to properly
refuse to change the thread desktop when it's in use.

diff --git a/server/thread.c b/server/thread.c
index 819fd9f..46d1e6e 100644
--- a/server/thread.c
+++ b/server/thread.c
@@ -140,6 +140,7 @@
     thread->suspend         = 0;
     thread->creation_time   = time(NULL);
     thread->exit_time       = 0;
+    thread->desktop_users   = 0;
 
     list_init( &thread->mutex_list );
     list_init( &thread->system_apc );