ssl: Add DTLS handshake primitivs.

This code is to 99 % written by Andreas Schultz only some small changes
to start integrating with OTPs DTLS solution.
ssl:  Refactor to provide common handshake functions for TLS/DTLS

Common functions will be located in ssl_handshake.erl while
specific functions will be located in tls_handshake.erl and dtls_handshake.erl
ssl: Add DTLS record primitives

This code is to 99 % written by Andreas Schultz only some small changes
to start integrating with OTPs DTLS solution.
ssl: DTLS record handling

Also refactor so that TLS and DTLS can have common functions when possible.
ssl: Solve rebase issues
ssl: Dialyzer fixes
ssl: Refactor TLS/DTLS record handling
Merge remote-tracking branch 'upstream/maint'
ssl: Prepare for release
Merge remote-tracking branch 'upstream/maint'