MFV r315791: ntp 4.2.8p10.

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