Add support for user avatars in the core protocol

Add a protocol and the APIs to straightforwardly support user avatars
in client applications. The protocol is designed to transfer avatars
in background, between friends only, and minimize network load by
providing a lightweight avatar notification for local cache validation.
Strict safeguards are imposed to avoid damage from non-cooperative or
malicious users and to limit network usage.

The complete documentation is available in docs/Avatars.md and sample
code is available in testing/test_avatars.c.

Code and documentation are released under the GNU GPLv3 or later, as
described in the file COPYING.