Add --storage-opt daemon option and some devicemapper option

This adds the --storage-opt commandline option to the docker daemon, additionally it adds a set of
configuration options for the devicemapper backend:

dm.basesize: Size of the base dm device (10G by default)
dm.loopdatasize: Initial size of the data looback file
dm.loopmetadatasize: Initial size of the metadata loopback file
dm.fs: The filesystem to use for the base image (xfs or ext4)
dm.mkfsarg: Add an argument to the mkfs command for the base image
dm.mountopt: Add a mount option for devicemapper mount
dm.datadev: Set raw block device to use for data
dm.metadatadev: Set raw block device to use for metadata
dm.blkdiscard: Disable the discard workaround (enabled by default if specifying raw disk devices)