ntp: import ntp-4.2.8p16

Security: NtpBUg3767, NtpBug3808, NtpBug3807 (CVE-2023-26555)
MFC after: immediately

MFV r293415:

ntp 4.2.8p5

Reviewed by: cy, roberto
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D4828

MFV ntp 4.2.8p1 (r258945, r275970, r276091, r276092, r276093, r278284)

Thanks to roberto for providing pointers to wedge this into HEAD.

Approved by: roberto