libevent: Import libevent 2.1.12Merge commit '5223d1d95fddcef6f9a36e264a5800bd907ade8b' into main(cherry picked from commit b50261e21f39a6c7249a49e7b60aa878c98512a8)
Copy libevent sources to contribTo replace the libevent embedded in ntp, for OpenSSL 1.1.1 compat.Approved by: re (gjb)