Speed improvements via code cleanup, and changes to make go dns behave more like glibc resolver.  See https://groups.google.com/forum/#!topic/golang-dev/lV-0aHqxVeo

Fixes #6579.

Benchmark results on linux/amd64

benchmark                                  old ns/op    new ns/op    delta
BenchmarkGoLookupIP                          4831903      2572937  -46.75%
BenchmarkGoLookupIPNoSuchHost               10114105      2419641  -76.08%
BenchmarkGoLookupIPWithBrokenNameServer  20007735624   5004490730  -74.99%

benchmark                                 old allocs   new allocs    delta
BenchmarkGoLookupIP                              287          288    0.35%
BenchmarkGoLookupIPNoSuchHost                    204          102  -50.00%
BenchmarkGoLookupIPWithBrokenNameServer          410          358  -12.68%

benchmark                                  old bytes    new bytes    delta
BenchmarkGoLookupIP                            13181        13271    0.68%
BenchmarkGoLookupIPNoSuchHost                  17260         8714  -49.51%
BenchmarkGoLookupIPWithBrokenNameServer        28160        22432  -20.34%

LGTM=mikioh.mikioh
R=golang-codereviews, mikioh.mikioh, bradfitz, josharian, abursavich
CC=golang-codereviews
https://golang.org/cl/128820043