rx: add and export a public keepalive toggle

rx: add and export a public keepalive toggle

make enabling and disabling keepalives a public function.
export the function

(cherry picked from commit 2a31f35936698c504c863702ebb675ac9dfe47e1)
Reviewed-on: http://gerrit.openafs.org/6517
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Change-Id: If7bd2b72980dd92771614a6d73a04441222a8314
Reviewed-on: http://gerrit.openafs.org/6522
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>