Enable various DTLS extensions.

Some TLS extensions were disabled for DTLS. Possibly because they caused
problems with the old duplicated code. Enable them again.
Extend DTLS method macros.

Extend DTLS method creation macros to support version numbers and encryption
methods. Update existing code.
Remove versions test from dtls1_buffer_message

Since this is always called from DTLS code it is safe to assume the header
length should be the DTLS value. This avoids the need to check the version
number and should work with any version of DTLS (not just 1.0).
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.