dtls: send psk server hint

If the user provides a psk server hint send it to the client.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: parse psk server identity hint

This is send in a server key exchange and is now parsed and forwarded
to the user.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: check length before parsing client identity

instead of just taking the first 32 bytes, check the length and reject
it if it is too long.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: check identity length in client key exchange

Before sending a client identity check if we have enough space for it.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: psk identity is now supported, remove the FIXMEs

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: put ecdsa and psk stuff into a union

This should save 32 bytes.

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