advisory includes a change which rejects handshakes with DH parameters
below 768 bits. sendmail releases prior to 8.15.2 (not yet released),
defaulted to a 512 bit DH parameter setting for client connections.
This commit chages that default to 1024 bits. sendmail 8.15.2, when
released well use a default of 2048 bits.
MFC after: 1 day
