dtls: use dissector for ServerHello extensions from ssl

DTLS is just a modification to TLS, the RFC only defines differences.
There are no big differences and it is possible to use the same
function to dissect the ServerHello message for both protocols. Use the
dissector from ssl also for the dtls handshake message.

This also adds support for better decoding some extensions.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>