net: use contexts for cgo-based DNS resolution

net: use contexts for cgo-based DNS resolution

Although calls to getaddrinfo can't be portably interrupted,
we still benefit from more granular resource management by
pushing the context downwards.

Fixes #15321

Change-Id: I5506195fc6493080410e3d46aaa3fe02018a24fe
Reviewed-on: https://go-review.googlesource.com/22961
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>