Fold dtls1_set_message_header into dtls1_set_handshake_header.

The frag_off/frag_len parameters are always zero, and the return value is never
used.

Change-Id: If7487b23c55f2a996e411b25b76a8e1651f25d8b
Reviewed-on: https://boringssl-review.googlesource.com/2601
Reviewed-by: Adam Langley <agl@google.com>