remus: init sch_plug module based on kernel version

remus: init sch_plug module based on kernel version

remus: init sch_plug module based on kernel version

sch_plug module, for network buffering, is available as part of linux
kernel (from 3.4 onwards), as opposed to an out-of-tree module.
The netlink message format to talk to the in-kernel module is different from
that of the old version.  So, before initializing the Plug Qdisc, check
the kernel version and use the appropriate message format.

Also change the names of the constants to reflect the format used by the mainline
module [CHECKPOINT -> BUFFER , RELEASE -> RELEASE_ONE ].

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>