libdm: Implement zero and linear targets.

This change implements DmTargetZero and DmTargetLinear, and integrates
them into dmctl. It also implements DmTarget and DmTable serialization.

Example dmctl invocation:

  dmctl create my-device -ro \
        linear 0 800 /dev/block/by-name/system 0 \
        zero   800 1200                          \
        linear 1200 1500 /dev/block/by-name/system 1200

Bug: 110035986
Test: libdm_test gtest
Change-Id: I7f945c1d9e23cfb78239c23a1aad88e8aef4972b