Application data MTU not exposed

[BrianSipos]

DTLS 1.2 in RFC 6347 recommends that the DTLS implementation expose to the application layer the effective application data MTU given the record/ciphersuite overhead. The openssl function to do this is DTLS_get_data_mtu which is not currently part of the this API.
I do see API for the application to hint to openssl what the underlayer network PMTU is.