ntp: Vendor import of ntp-4.2.8p18

MFC: 3 days

Merge commit '1f833b3fc9968c3dd7ed79ccf0525ebf16c891ad' into main

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