Fix slow shutdowns on error

SDL_threadID was being compared to int and woudl thus always fail,
resulting in the application thinking that another thread was trying to
shutdown.

Source/appfat.cpp

@@ -6,7 +6,8 @@ DEVILUTION_BEGIN_NAMESPACE
 
 char sz_error_buf[256];
 BOOL terminating;
-int cleanup_thread_id;
+/** Thread id of the last callee to FreeDlg(). */
+SDL_threadID cleanup_thread_id;
 
 void app_fatal(const char *pszFmt, ...)
 {

Source/appfat.h

@@ -4,7 +4,6 @@
 
 extern char sz_error_buf[256];
 extern BOOL terminating;
-extern int cleanup_thread_id;
 
 void TriggerBreak();
 char *GetErrorStr(DWORD error_code);