RFC: message property API

[David Bremner]

People (or maybe just dkg) have proposed various bits of user metadata
to attach to messages

   - references to be added or blacklisted
   - encryption related key material

Since such metadata would be not-reproducible from the message files,
it would be "precious" and need a backup/restore path.

This is a prototype of a library API to support that.
    
Some things to note:
    
    - This is tested on top of my libconfig [1] series, but I think there is no
      logical dependency
    
    - there are no docs, see the tests
    
    - this is just the low level api; it would need to be extended in a
      similar way to the libconfig series to dump and restore metadata.
      This might require modifying/extending the API to dump all properties
      for a message. That should be a relatively small change.
    
    - it also be useful to be able delete all (key,value) pairs with a given
      key.
    
    - this should really go through the existing caching mechanism used by
      e.g. tags, or at least not invalidate that cache.
    
[1] id:1463135893-7471-6-git-send-email-david at tethera.net