dtls: do not expect an other fragment every time

The old code expected a new fragment every time, now when the last part
is send we do not expect an other one. This will not work if we get the
packages in an other order.

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