Add dm-snapshot targets to libdm and dmctl.

This adds DmTargetSnapshotOrigin and DmTargetSnapshot. The latter target
can handle both "snapshot" and "snapshot-merge" targets. The syntax for
dmctl is as follows:

    dmctl create <name> snapshot <start> <num_sectors> <base_device> \
          <cow_device> <P|N> <chunk_size>
    dmctl create <name> snapshot-merge <start> <num_sectors> <base_device> \
          <cow_device> <chunk_size>
    dmctl create <name> snapshot-origin <start> <num_sectors> <device>

Bug: N/A
Test: libdm_test gtests
Change-Id: I8eef987cb92121e81bedd37b9a66fad04c7a23a3