Fix return value of b64encode().
Use Ed25519 keys.

This uses the portable Ed25519 library made by Orson Peters, which in turn uses
the reference implementation made by Daniel J. Bernstein.

This implementation also allows Ed25519 keys to be used for key exchange, so
there is no need to add a separate implementation of Curve25519.
Merge branch '1.1-ed25519' into 1.1
Use the ChaCha-Poly1305 cipher for the SPTPS protocol.

The main reason to switch from AES-256-GCM to ChaCha-Poly1305 is to remove a
dependency on OpenSSL, whose behaviour of the AES-256-GCM decryption function
changes between versions. The source code for ChaCha-Pol1305 is small and in
the public domain, and can therefore be easily included in tinc itself.
Moreover, it is very fast even without using any optimized assembler, easily
outperforming AES-256-GCM on platforms that don't have special AES instructions
in hardware.