ssl: add missing valid handshake types

The DTLS dissector duplicated a handshake types check, this has been
eliminated. Convert HandshakeType and ContentType to enums to get the
benefit of compiler-checked switch cases. Move these checks to
ssl-utils.

Two default cases could never be reached since the dissector returns
immediately on an invalid ContentType.

Also fixed misleading debugging messages.

Change-Id: I07a2062564e073004dcc0401cd82538e5659fa0c
Reviewed-on: https://code.wireshark.org/review/2978
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Reviewed-by: Anders Broman <a.broman58@gmail.com>