This change replaces the vendored socks client implementation with the
bundle of golang.org/x/net/internal/socks package which contains fixes
for 19354 and 11682.

golang.org/x/net/internal/socks becomes socks_bundle.go.

At git rev 61147c4. (golang.org/cl/38278)

Updates #11682.
Updates #18508.
Updates #19354.
Fixes #19688.
Updates #21333.

Change-Id: I8cf6c3f5eb87c24685a7592be015729f84fbed77
Reviewed-on: https://go-review.googlesource.com/41031
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>