The interface to set TCP keepalive on Plan 9 is
writing the "keepalive n" string to the TCP ctl file,
where n is the milliseconds between keepalives.

Fixes #11266.

Change-Id: Ic96f6c584063665a1ddf921a9a4ddfa13cc7501b
Reviewed-on: https://go-review.googlesource.com/11860
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>