Implement control socket for DDNS

After the design (#3540) is done, we should implement control socket in DDNS server.

D2, our DHCP-DDNS update daemon, does not support control channel. CA does support it and has a section for D2 communication, tries to open a socket etc, but fails ultimately, because D2 is not able to listen on that socket.

The absolute minimum required are the following commands:

version-get
build-report
shutdown 

Since it is unclear whether we'll be able to squeeze this into 1.3, adding this with low priority.