net: pass MSG_CMSG_CLOEXEC flag in ReadMsgUnix

As mentioned in #42765, calling "recvmsg" syscall on Linux should come
with "MSG_CMSG_CLOEXEC" flag.

For other systems which not supports "MSG_CMSG_CLOEXEC". ReadMsgUnix()
would check the header. If the header type is "syscall.SCM_RIGHTS",
then ReadMsgUnix() would parse the SocketControlMessage and call each
fd with "syscall.CloseOnExec"

Fixes #42765

Change-Id: I74347db72b465685d7684bf0f32415d285845ebb
GitHub-Last-Rev: ca59e2c9e0e8de1ae590e9b6dc165cb768a574f5
GitHub-Pull-Request: golang/go#42768
Reviewed-on: https://go-review.googlesource.com/c/go/+/272226


Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>