Merge pull request #6911 from ctiller/idempotent_endpoint_shutdown

src/core/lib/iomgr/endpoint.h

src/core/lib/iomgr/ev_poll_and_epoll_posix.c

src/core/lib/iomgr/ev_poll_posix.c

src/core/lib/iomgr/ev_posix.c

src/core/lib/iomgr/ev_posix.h

src/core/lib/iomgr/tcp_posix.c

test/core/iomgr/endpoint_tests.c

test/core/iomgr/tcp_posix_test.c