[FR] Introduce libunbound-control library for external consumers

[rcmcdonald91]

Current behavior
Currently all the business logic for unbound-control is self-contained within unbound-control.c.

Describe the desired feature
Break out this core functionality into a shared library (say libunbound-control) and reimplement unbound-control in terms of this library.

Potential use-case
The ability for external consumers to link against libunbound-control and communicate with Unbound in a variety of ways.

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We should also start a conversation about the design of this library. My initial thought is to start moving functionality from unbound-control into libunbound. I don't think we strictly need another library when we already have libunbound and the requisite build infrastructure.

[rcmcdonald91]

I have a functional PoC

One thing that I would like is to be able to recycle the connection, SSL context between requests. Any advice here would be most welcomed. It doesn't look like daemon/remote.c is able to handle this.

[rcmcdonald91]

#1058