LsfSocket, SetLsfPromisc and NetlinkRIB currently don't force the CLOEXEC
flag on the sockets they create. While the former two functions are
deprecated, NetlinkRIB is called by various functions related to
net.Interface.

Add a helper to create CLOEXEC sockets, and use it from SetLsfPromisc and
NetlinkRIB. LsfSocket is unchanged since we don't want to break callers.

Fixes #36053

Change-Id: I72fe2b167996797698d8a44b0d28165045c42d3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/210517
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>