libevent: Import libevent 2.1.12MFC after: 2 weeksMerge commit '5223d1d95fddcef6f9a36e264a5800bd907ade8b' into main
Copy libevent sources to contribTo replace the libevent embedded in ntp, for OpenSSL 1.1.1 compat.Approved by: re (gjb)