OpenFlow 1.0: Simplify the snapend management. [skip ci]

OpenFlow 1.0: Simplify the snapend management. [skip ci]

As Guy explains it, nd_push_snapend() makes it easy to save and to
restore the snapend from different functions or different invocations
of the same function. In a simple case a local variable and ND_MIN()
do the job just as well without the associated memory management and
error checking.