dtls: document Subject Public Key ASN.1

This also adds a check to compare the received ASN.1 header with the
expected one in the parsing of the Certificate message.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: use hex numbers to encode ASN.1 types

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