Fix reporting of DLL unload events on MS-Windows.

 gdb/windows-nat.c (handle_unload_dll): Don't call solib_add for the
 unloaded DLL, it will be done by handle_solib_event.  See
 http://sourceware.org/ml/gdb-patches/2013-05/msg00713.html for the
 details.
	* gdb.threads/wp-replication.c (main): Insert some code at the start
	to ensure the breakpoint on main is only hit once.  Fix comment.
	* spinlock.h (ULONG): Replace LONG operator with ULONG to accommodate
	the fact that CURR_SHARED_MAGIC and USER_SHARED_MAGIC are unsigned
	values.
	* shared.cc (shared_info::initialize): Drop explicit cast here.