Suggested by: @oparoz

Spotted by: @fabianfrz

We don't use this file very often so clamp down the access range
in order to avoid unwanted leakage.

The two are mutually exclusive.

o When servers are empty, show prefer and noselect as disabled.
o Tighten the config write by using the same code idiom as the
  frontend, which is the safer method.
o Untangle noselect and prefer, ntpd.conf will handle it.