Bug#37226 Explicit call of my_thread_init() on Windows for every new thread.

Bug#33031 app linked to libmysql.lib crash if run as service in vista under 
localsystem
  

There are some problems using DllMain hook functions on Windows that 
automatically do global and per-thread initialization for libmysqld.dll

1)per-thread initialization(DLL_THREAD_ATTACH)
MySQL internally counts number of active threads that and causes a delay in in 
my_end() if not all threads are exited. But,there are threads that can be 
started either by Windows internally (often in TCP/IP scenarios) or by user 
himself - those threads are not necessarily using libmysql.dll functionality, 
but nonetheless the contribute to the count of open threads.

2)process-initialization (DLL_PROCESS_ATTACH)
my_init() calls WSAStartup that itself loads DLLs and can lead to a deadlock in 
Windows loader.

Fix is to remove dll initialization code from libmysql.dll in general case. I
still leave an environment variable LIBMYSQL_DLLINIT, which if set to any value 
will cause the old behavior (DLL init hooks will be called). This env.variable 
exists only to prevent breakage of existing Windows-only applications that 
don't do mysql_thread_init() and work ok today. Use of LIBMYSQL_DLLINIT is 
discouraged and it will be removed in 6.0

libmysql/dll.c

@@ -89,9 +89,20 @@ BOOL APIENTRY LibMain(HANDLE hInst,DWORD ul_reason_being_called,
   UNREFERENCED_PARAMETER(lpReserved);
 } /* LibMain */
 
+
+static BOOL do_libmain;
 int __stdcall DllMain(HANDLE hInst,DWORD ul_reason_being_called,LPVOID lpReserved)
 {
-  return LibMain(hInst,ul_reason_being_called,lpReserved);
+  /*
+    Unless environment variable LIBMYSQL_DLLINIT is set, do nothing.
+    The environment variable is checked once, during the first call to DllMain()
+    (in DLL_PROCESS_ATTACH hook).
+  */
+  if (ul_reason_being_called == DLL_PROCESS_ATTACH)
+    do_libmain = (getenv("LIBMYSQL_DLLINIT") != NULL);
+  if (do_libmain)
+    return LibMain(hInst,ul_reason_being_called,lpReserved);
+  return TRUE;
 }
 
 #elif defined(WINDOWS)