This commit adds the js/wasm architecture to the net package.
The net package is not supported by js/wasm, but a simple fake
networking is available so tests of other packages that require
basic TCP sockets can pass. The tests of the net package itself
are mostly disabled.

Updates #18892

Change-Id: Id287200c39f0a3e23d20ef17260ca15ccdcca032
Reviewed-on: https://go-review.googlesource.com/109995
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>