Provide functions for starting, finishing and writing SSL handshake

Provide functions for starting, finishing and writing SSL handshake
messages. This will allow for removal of repeated/duplicated code.

Additionally, DTLS was written by wholesale copying of the SSL/TLS code,
with some DTLS specifics being added to the duplicated code. Since these
SSL handshake message functions know how to handle both SSL/TLS and DTLS,
upon conversion the duplicate versions will become identical (or close to),
at which point the DTLS versions can be removed and the SSL/TLS versions
used for both protocols.

Partially based on similar changes in OpenSSL.

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