Provisional DTLS 1.2 support.

Add correct flags for DTLS 1.2, update s_server and s_client to handle
DTLS 1.2 methods.

Currently no support for version negotiation: i.e. if client/server selects
DTLS 1.2 it is that or nothing.
(cherry picked from commit c3b344e3)

Conflicts:

	apps/s_server.c