dtls: do not expect ECC TLS extensions in ServerHello

RFC4492 does not say that the server should send a Supported Elliptic
Curves Extension and the Supported Point Formats Extension should only
be send when other formats expect uncompressed format are supported. In
the ServerHello we should not expect both Extensions.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: do not dump cleartext if decryption failed

The pointer and the length are not valid when the decryption failed.
This will cause wrong memory accesses.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
dtls: improve comment for DTLS_MAX_BUF

For some DTLS clients the DTLS_MAX_BUF has to be increase. This is
needed for Californium with ECDSA.

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