increment the sheep protocol version

Because of the changing struct sd_node layout, dog of v0.7.x cannot
communicate with the latest sheep. This patch forbid the mismatch
communication by incrementing SD_SHEEP_PROTO_VER.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: update log level help

We cannot specify a number as a log level.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>