[Nix-dev] Enable openntpd instead ntp by default

[Peter Simons]

Minor side note: I tried to update the ntp package to version 4.2.8, but
the build fails when --without-crypto is specified (like we do). Duh. :-(

Apparently, the latest vulnerability applies only to instances that
*have* crypto code enabled, it our moderately old copy of ntp isn't
vulnerable anyway.

Anyway ... +1 for switching to some other, simpler solution by default.

Peter