corosync: fix a compiler warning

Fix following warning:

...
cluster/corosync.c: In function ‘corosync_get_local_addr’:
cluster/corosync.c:132:22: warning: declaration of ‘sin’ shadows a global declaration [-Wshadow]
  CCLD   sheep
make[1]: Leaving directory `/home/test/sheepdog/sheep'
...

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
export prepare_iocb for future use

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: introduce file store

File store maps the volume directly to the file and stores it as a plain file.

This allows the use case that wants to store the user data as the file and use
the sheep as a standalone daemon instead of a cluster, similar to QEMU NBD.

QEMU --> sheep --> files

Since there is only one sheep daemon, we don't consider recovery logic.

Usage:
$ dog cluster format -b file -c 1

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
file store: add delete operation

Unlike plain store, we remove the files from file store by 'dog vdi delete' if
there is no successive vdi in the bitmap.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>