Features
   * The new functions mbedtls_dhm_get_len() and mbedtls_dhm_get_bitlen()
     query the size of the modulus in a Diffie-Hellman context.
   * The new function mbedtls_dhm_get_value() copy a field out of a
     Diffie-Hellman context.

API changes
   * Instead of accessing the len field of a DHM context, which is no longer
     supported, use the new function mbedtls_dhm_get_len() .