Rehabilitate pss handshakes

[nolash]

Currently, pss handshake tests are deactivated. There is no guarantee that they work as intended.

That said, the handshake implementation seems somewhat haphazard, and it is questionable if it's at all within the scope of pss to provide such a handshake mechanism. And even if it is, it should still make it possible to interface with existing and well tested negotiation and exchange mechanisms for session keys.

This suggests that:

• handshakes should be a separate package
• handshakes should provide an interface to hook in existing session key implementations
• existing session key implementation should implement said interface
• should probably be renamed to session