ndb/dns: ignore terminating authoritative flag for no-answer when more nameservers are provided

continue recursing when we get empty but non-negative answer
from a (claimed) authoritative nameserer that provides more
nameservers.

this fixes wordpress dns:

63766.3: sending to 192.0.80.93/ns1.wordpress.com bossypally.files.wordpress.com ip
63766: rcvd OK from 192.0.80.93 (authoritative)
	Q:    bossypally.files.wordpress.com ip
	Auth: files.wordpress.com              5 min           ns   	mdns1.wordpress.com
	      files.wordpress.com              5 min           ns   	mdns2.wordpress.com
	      files.wordpress.com              5 min           ns   	mdns3.wordpress.com
	      files.wordpress.com              5 min           ns   	mdns4.wordpress.com
	      files.wordpress.com              5 min           ns   	mdns5.wordpress.com
	Hint: mdns1.wordpress.com              4 hr            ip   	192.0.75.7
	      mdns2.wordpress.com              4 hr            ip   	198.181.117.7
	      mdns3.wordpress.com              4 hr            ip   	198.181.116.7
	      mdns4.wordpress.com              4 hr            ip   	198.181.118.7
	      mdns5.wordpress.com              4 hr            ip   	192.0.74.7


63766.4: sending to 192.0.75.7/mdns1.wordpress.com bossypally.files.wordpress.com ip
63766: rcvd OK from 192.0.75.7 (authoritative)
	Q:    bossypally.files.wordpress.com ip
	Ans:  bossypally.files.wordpress.com   5 min           ip   	192.0.72.2
	      bossypally.files.wordpress.com   5 min           ip   	192.0.72.3
----------------------------
answer bossypally.files.wordpress.com   5 min           ip   	192.0.72.2
answer bossypally.files.wordpress.com   5 min           ip   	192.0.72.3
----------------------------

note the authoritative flag in the first response from ns1.wordpress.com that
would otherwise terminate the search.
ethervirtio: feature negotiation, allow setting mac address, dont read isr from ifstatus, avoid indirection

- properly negotiate Fctrlrx feature bit for promisc and multicast.
- allow setting mac address with ea= option from plan9.ini
- dont read the isr register from ifstats() as it has the side effect of reseting isr status
- embedd the Vqueue array in the Ctlr structure avoiding indirection
- add a interrupt counter Vqueue.nintr for statistical purposes
- only read network status register if the feature has been negotiated
- change name to "virtio" as "ethervirtio" is kind of redundant
hgwebfs: add fake add_handler method for HTTPRangeHandler()

provide a add_handler() so code trying to add
byterange.HTTPRangeHandler will succeed. webfs
does handle "206 Partial Content" already.