net: support context cancellation in resSearch
As with all the stuff that call cgo from net package.
Change-Id: I7c42ae44a1d47f4f949b203682217498fcdba92a
GitHub-Last-Rev:
70406493bbbe10bf556a17e453623d3decf00822
GitHub-Pull-Request: golang/go#57043
Reviewed-on: https://go-review.googlesource.com/c/go/+/454697
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>