ndns-dig: adapt to the fact that NDN testbed does not have root zone

- Enable dig to set the start label during iterative process.
- Enable validation of intermediate results with an option to disable
it

Change-Id: I221e3c328c875ad06a1f8094f1004e68d9d46a57
tools: Fix issues with ndns-dig

- Handle the case when validator.conf is not present and anchors cannot
  be found (default path for anchor lookups is $SYSCONFDIR/ndns)

- Abort ndns-dig when positional parameters are omitted

refs: #2276

Change-Id: Id70d4b219254f5b221990a38d41471b920aa1521
ndns-update: load Response from file directly

- also fix the bug that ndns-update segfaults when running without
parameters

Change-Id: I65a741e845d44d14aa29b2814c6616d27dc0a12a
Refs: #2255, #2273