ovs_threads: Avoid running pthread destructors from main thread exit.

ovs_threads: Avoid running pthread destructors from main thread exit.

Windows uses pthreads-win32 library to provide the Linux pthread
functionality. It is observed that when the main thread calls
a pthread destructor after it exits, undefined behavior is seen
(e.g., junk values in data, causing pthread deadlocks).
Similar behavior has been seen by
other people as seen in the following email thread:
https://sourceware.org/ml/pthreads-win32/2003/msg00001.html

To avoid this, this commit de-registers the thread destructor
when the main thread exits (via the atexit handler).

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>