[devmgr] Use C++ enums for message ops Test: cq Change-Id: I2e32fd14b1be11b5e238b5460d6a5c2c6cc3d981
[devmgr] Prefer nullptr to NULL in C++ Test: CQ Change-Id: Ia76dc1cc448c393208b1426ed66c90054a1308a3
[core] Build dmctl as C++ Test: CQ Change-Id: Iebd2c052e00b9c32bc1527fa4caaf68891fa1122