Turn off include_optional_ns for gdnsd

This switches us to gdnsd default behavior of not
including an Authority section (or the relevant Additional
records) in most typical responses where the protocol
doesn't require it.

The SOA-record authority section is still always included
for e.g. NXDOMAIN and no-data responses, and NS records
(and their additionals) are still copied in for referrals.

This significantly reduces DNS response packet sizes in
common cases.  It's a 64 byte reduction for a typical
query on en.wp.o (142 -> 78) and up to ~100 byte reduction
in some other cases, out of a traditional limit of 512.
This has a number of positive fallouts for things like
traffic rates, likelihood of fragmentation or TCP
fallback for larger (e.g. anti-spam TXT) responses,
authserver efficiency under attack, anti-reflection, etc.

There's no real downside that I see.  Caches will have to
explicitly refresh our NS data from us or upstream TLD
servers more-frequently, but this is outweighed by many
orders of magnitude by the reduction in normal traffic.

For those that are fans of argument from authority, Google
does this optimization too :p

Example change in query response (example is on top of the
scenario where the DYNA changes in Iefae74d8d are already
deployed, which already reduces response size considerably):

For en.wp.o:

OLD behavior:---------------------------------------------
$ dig -p 53535 @ns1.wikimedia.org en.wikipedia.org
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 3, ADDITIONAL: 1
;; ANSWER SECTION:
en.wikipedia.org.	600	IN	A	208.80.154.224
;; AUTHORITY SECTION:
wikipedia.org.		86400	IN	NS	ns1.wikimedia.org.
wikipedia.org.		86400	IN	NS	ns2.wikimedia.org.
wikipedia.org.		86400	IN	NS	ns0.wikimedia.org.
;; ADDITIONAL SECTION:
en.wikipedia.org.	600	IN	AAAA	2620:0:861:ed1a::1
;; MSG SIZE  rcvd: 142

NEW behavior:---------------------------------------------
$ dig -p 53535 @ns1.wikimedia.org en.wikipedia.org
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; ANSWER SECTION:
en.wikipedia.org.	600	IN	A	208.80.154.224
;; ADDITIONAL SECTION:
en.wikipedia.org.	600	IN	AAAA	2620:0:861:ed1a::1
;; MSG SIZE  rcvd: 78

Change-Id: I65008db482ad6a8ea6774169a33948d87307952d