Fix for double CloseHandle() on AppDomain unload

Fix for double CloseHandle() on AppDomain unload

When AppDomain.Unload() is called a new thread is created and attached. When
this thread is done the thread handle is immediately closed using
CloseHandle() but the same handle is also passed to CloseHandle() later on by
the InternalThread destructor. On the second CloseHandle() call Windows will
abort the process with an "Invalid handle" error.

This patch removes the CloseHandle() calls in mono_domain_try_unload() which
is called by AppDomain.Unload(). The patch also wires up some more functions
in mono/utils/mono-threads-windows.c to set the MonoThreadInfo.handle field to
NULL when a thread has exited.