Finally fix __yp_ping(). We can't use the old locally defined clntudp_call()

Finally fix __yp_ping(). We can't use the old locally defined clntudp_call()
method anymore since the code inside the RPC library has changed too much.
Now that the clnt_dg module has the necessary code internally, we can yank
out the local method code and turn on the ASYNC hack with clnt_control().
This will make the -m flag work again.