sheep: prevent starvation of "node kill" requests

Under heavy load, sheep process continues to execute event_loop()
even if admins execute "dog node kill" because
sys->nr_outstanding_reqs rarely reaches 0. In our case, actual
exit of sheep process took about from 10 min to 1 hour to complete.

This patch prevents the starvation of the stop requests by
unregistering events of listening fds. By unregistering, sheep can
stop generating new requests.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: add a new option "-l" to node kill for killing local sheep process

This patch adds a new option "-l" to "dog node kill". The purpose of
this option is killing a sheep process which runs on local host. It is
convenient for terminating sheep process by scripts like
/etc/init.d/sheepdog because the scripts don't have to know about the
ID of the sheep process.

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