app/testpmd: add help messages for multi-process

app/testpmd: add help messages for multi-process

This patch adds help messages for multi-process.
--num-procs=N: set the total number of multi-process instances.
--proc-id=id:  set the id of the current process from multi-process
instances(0 <= id < num-procs).

Fixes: a550baf24af9 ("app/testpmd: support multi-process")
Cc: stable@dpdk.org
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Signed-off-by: Min Hu (Connor) <humin29@huawei.com>
Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
Acked-by: Ferruh Yigit <ferruh.yigit@xilinx.com>