Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Update our ntpd to the long awaited 4.2.0 version.

This bring us several things:
- updated drivers
- IPv6 support at last
- ntp-genkeys is replaced by ntp-keygen
- ntptrace is now a script (courtesy of John Hay)
- lots of renamed files from .htm to .html (while I prefer .html, I
find the change a bit gratuitous)
- still no manpages :(

Please test and report.

Commit very much helped by: GNU arch (http://gnuarch.org/)