sheep: don't start recovery if cluster was shutdowned

Add a new field in the config file to indicate shutdown status. The layout of
config file isn't modified, just find a free slot to hold shutdown status.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
sheep: remove unused get_max_copy_number()

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
make: add 'check-ununsed' command

'make check-unused' will try to find the possible unused functions that is
defined but used functions.

E.g

yliu@ubuntu-precise:~/sheepdog$ make check-unused
recalculate_vnodes
object_cache_try_to_reclaim
default_cleanup
default_create_and_write
default_exist
default_format
default_get_hash
default_init
default_link
default_purge_obj
default_read
default_remove_object
default_update_epoch
default_write
get_max_copy_number
...

functions listed above are either
1) defined but not used by other c files, indicating that we should define them
static functions
2) defined but not used at all, indicating that we should remove them completely

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
mark the functions static as much as possible

These functions are found by 'make check-unused' and this patch doesn't try to
deal with default_xxx functions in plain.c and object_tree_print, which are
expected to be called by future users.

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