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.

The new package targets have no drawbacks compared to auto live
mount, but the latter has proven difficult and does not match the
normal boot scenario and could potentially panic due to unionfs
being broken beyond repair (kib@'s words in spirit, not mine).

Discussed with: @adschellevis