[3.6] bpo-37228: Fix loop.create_datagram_endpoint()'s usage of SO_REUSEADDR (GH-17311). (GH-17571)

(cherry picked from commit ab513a38c98695f271e448fe2cb7c5e39eeaaaaf)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>

• Doc/library/asyncio-eventloop.rst[diff]
• Lib/asyncio/base_events.py[diff]
• Lib/test/test_asyncio/test_base_events.py[diff]
• Misc/NEWS.d/next/Security/2019-11-21-21-36-54.bpo-37228.yBZnFG.rst[Added - diff]