net: remove non-cgo macOS resolver code

The built-in Go resolver works significantly better.
In particular, the use of res_search does not support
CNAME or PTR queries and may not even be thread-safe.

This CL is essentially a revert of CL 166297 plus fixes,
including CL 180842.

See CL 180842 for additional notes about problems
with this approach.

Fixes #31705.

Change-Id: I0a30a0de2fbd04f6c461520fd34378c84aadf66c
Reviewed-on: https://go-review.googlesource.com/c/go/+/180843


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>