Dedup DTLS header writing code and convert to CBB.

Dedup DTLS header writing code and convert to CBB.

There are three versions of the DTLS header writing code, which primarily
differ by the fragment offset and fragment length values that differ.
Rework dtls1_write_message_header() such that it can be used in all three
cases and convert it to CBB in the process.

ok inoguchi@ tb@