Environment variable 'ALL_PROXY=socks5h://example.com' is commonly
used to specify a SOCKS5 proxy server.
In curl, 'socks5' means the host name will be resolved locally,
and 'socks5h' means the host name will be resolved by the server.

Go SOCKS5 client always uses the server to resolve host names.
So this change just added socks5h as a supported URL scheme.

Fixes golang/go#13454

Change-Id: I06d2b07f66cd0923c114dba4df0f884b39e58bc0
Reviewed-on: https://go-review.googlesource.com/c/156517Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>