This change factors out the code related to SOCKS protocol version 5
from the golang/x/net/proxy package and provides new SOCKS-specific
API to fix the following:
- inflexbility of forward proxy connection setup; e.g., no support for
  context-based deadline or canceling, no support for dial deadline,
  no support for working with external authentication mechanisms,
- useless error values for troubleshooting.

The new package socks is supposed to be used by the net/http package
of standard library and proxy package of golang.org/x/net repository.

Fixes golang/go#11682.
Updates golang/go#17759.
Updates golang/go#19354.
Updates golang/go#19688.
Fixes golang/go#21333.

Change-Id: I24098ac8522dcbdceb03d534147c5101ec9e7350
Reviewed-on: https://go-review.googlesource.com/38278
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>