clock_id: These symbols weren't in 4.4BSD, adjust copyright

Peter Wemm added the first CLOCK_* symbols in 0f5ed9f420528 in 1997
after obtaining them from NetBSD. In NetBSD, jtc@netbsd.org committed
them in sys/sys/time.h rev 1.19 dated 1996/11/15, along with all the
system calls associated with 1003.1b. FreeBSD's values are, however,
different than NetBSD's today. The USL/UCB lawsuit was settled in 1994,
so these couldn't have been derived from material provided to University
of California covered in that settlement. This file does not need the
settlement disclaimer.

Furthermore, I rewrote most of the code (except the symbols and their
values) when merging it from time.h and sys/time.h. Most of the creative
content of the file is new, so update copyright to reflect that.

Reviewed by: kaktus
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D31369