sheep: check resource limit at startup

Sheep daemon is FD hungry and can't survive for EMFILE.
1024 is default for NOFILE on most distributions, which is very
dangerous to run Sheepdog cluster. Let's give a warning on this.

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>