ssl,dtls: fix CertificateVerify dissection for TLSv1.2

ssl,dtls: fix CertificateVerify dissection for TLSv1.2

TLSv1.2 gained an additional SignatureAndHashAlgorithm field for fields
marked with the digitally-signed attribute. This was already implemented
before for ServerKeyExchange, let's reuse that.

Note that the SignatureAndHashAlgorithm tree and fields (hash algo,
signature algo) are repurposed in a different context, but since the
structure is the same it is kept like this.

By the way, add support for DTLSv1.2 too. RFC 6347 section 4.2.6
suggests that the implementation is the same (as far as the dissector is
concerned).

Also update the comments and remove the additional "Signature with
client's private key" subtree since the CertificateVerify message has no
other items.

Bug: 11045
Change-Id: I025901b85e607f04d60357ff14187cc13db2ae5d
Reviewed-on: https://code.wireshark.org/review/7650
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Tested-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Michael Mann <mmann78@netscape.net>