Switch ntp's embedded libevent to 2.1.18

For OpenSSL 1.1.1 compatibility.

In Makefile.inc1 add (to the existing similar cases) a hack to handle
dependencies across the migration.

Reviewed by: jhb
Approved by: re (gjb)
Sponsored by: The FreeBSD Foundation.
Differential Revision: https://reviews.freebsd.org/D17481