dtls: use dtls_send() for sending a handshake message
dtls: use dtls_send() for sending a handshake message

This changes dtls_send() to dtls_send_multi() which is able to take an
array of buffers to send and not just one. This is now also used in
dtls_send_handshake_msg_hash() to send the handshake messages, because
dtls_send() also handles retransmission and handshake messages should
be retransmitted when needed.

Signed-off-by: Hauke Mehrtens <435ddd46dc66c007a1fa20144c823d37e0d23436@hauke-m.de>

jvermillard/tinydtls,nmacs/tinydtls,OlegHahm/tinydtls,yunus/TinyDTLS-WebID,jvermillard/tinydtls,spark/tinydtls,spark/tinydtls,OlegHahm/tinydtls,yunus/TinyDTLS-WebID,jvermillard/tinydtls,thecodemaiden/tinydtls-fork,thecodemaiden/tinydtls-fork,nmacs/tinydtls,thecodemaiden/tinydtls-fork,nmacs/tinydtls,nmacs/tinydtls,fvdnabee/tinydtls,spark/tinydtls,fvdnabee/tinydtls,fvdnabee/tinydtls,yunus/TinyDTLS-WebID,OlegHahm/tinydtls