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>
update CHANGELOG.md

Three changes:
1. output format of "dog vdi snapshot" with -v option
2. "dog vdi object" subcommand
3. fix invalid description of "dog node level"

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: allow snapshot tags which begin with decimal number

Current dog command doesn't accept snapshot tags which begin with
decimal number, e.g. 123abc. This behavior comes from invalid usage of
strtol(). This patch fixes this problem.

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>