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.
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