devmapper: zero the first 4K of the base thin device

Eliminates any potential for udev to identify stale labels on the base
device during its scanning.

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)
devmapper: Add option for specifying an lvm2 created thin-pool device

Docker will not activate/deactivate the specified thin-pool device but
it will exclusively manage/create thin and snapshot thin volumes in it.

Docker will not take ownership of the specified thin-pool unless it has
0 data blocks used and a transaction id of 0.  This should help guard
against using a thin-pool that is already in use.

Also fix typos in setupBaseImage().

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)