LU-2467 protocol: Add OBD_CONNECT_PINGLESS

Reserve a bit for OBD_CONNECT_PINGLESS, which indicates a client is
capable of suppressing keep-alive OBD_PINGs.  If granted by a server,
it means the server does not require (but still allows) pings.

Signed-off-by: Li Wei <wei.g.li@intel.com>
Change-Id: I7575082244a84f2e633a1936fcf894da3c6ee7dd
Reviewed-on: http://review.whamcloud.com/5229
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>