Add DTLS_get_data_mtu() function

We add ssl_cipher_get_overhead() as an internal function, to avoid
having too much ciphersuite-specific knowledge in DTLS_get_data_mtu()
itself. It's going to need adjustment for TLSv1.3... but then again, so
is fairly much *all* of the SSL_CIPHER handling. This bit is in the noise.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>