devmapper: Rename DeviceSet.RemoveDevice to DeleteDevice

"Remove" is a bit overloaded, as it is also what deactivating a
device mapper device is called. Using "delete" is more clear here.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: add and use removeDeviceAndWait helper

This adds a function that calls the lowlevel removeDevice
and then waits for it to finish.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Move refcounting to DeviceSet

We already have some kind of refcounting in DeviceSet, this fleshes
it out to allow it to completely subsume the refcounting in
devmapper.Driver.

This allows us to drop the double refcounting, and the locking inside
devmapper.Driver. This, in particular the locking simplification will
make it easier in the future to parallelize the device mapper.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Use removeDeviceAndWait in DeviceSet.removeDevice()

This makes sure the device is removed just like in deactivateDevice.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Handle EBUSY while removing

For some reason we seem to get transient EBUSY when removing
thinp devices, which prohibit removing containers. When
this happens we retry a few times which seems to fix the
issue for me.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Fix UnmountDevice for non-existing device

Properly error out if passed an id that doesn't exist.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
devmapper: Fix MountDevice for non-existing Device

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #3948 from alexlarsson/devmapper-unmount

Devmapper cleanup and unmount fix
Added Security FAQ security

Docker-DCO-1.1-Signed-off-by: James Turnbull <james@lovedthanlost.net> (github: jamtur01)
Merge pull request #3984 from jamtur01/secfaw

Added Security FAQ question