net: correct address when listening on IPv4zero tcp4/udp4 on Plan 9

Since Plan 9 doesn't allow us to listen on 0.0.0.0, the Listener
address that's read in from /net is the IPv6 address ::. Convert
this address to 0.0.0.0 when the network is tcp4 or udp4.

Fixes #40045

Change-Id: Icfb69b823e5b80603742d23c3762a812996fe43f
Reviewed-on: https://go-review.googlesource.com/c/go/+/240918


Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>