add MDP client library target to Makefile

[dismantl]

An MDP client library would be useful for software wanting to make use of the MDP overlay without needing the whole serval-dna library (e.g. I'm using the MDP client library in the Commotion Service Manager for fetching SAS keys).

[quixotique]

An MDP client library requires far more engineering work than merely adding a target to the Makefile. For example:

• All dependencies on daemon-only infrastructure like config and logging must be broken and replaced by other, programmer-customisable services on the client side.
• Foundation types like sid_t and rhizome_bid_t must be factored out of the serval.h and rhizome.h headers, and into their own headers that can be exposed to client-side programs. Commit 21f51965c18948cc5de8c71793df5dc41f6c743b is an important step towards this.
• The Monitor interface must be replaced by MDP control messages, so that the Monitor interface can be completely removed.
• The MDP control protocol needs to be formally specified and reviewed for forward/backward compatibility issues.
• The MDP API needs to be polished a bit and documented, with an eye to making it extensible.