dccp: Special case of the MPS for client-PARTOPEN with DataAcks

To increase robustness, it is necessary to resend Confirm feature-negotiation
options, even though the RFC does not mandate it. But feature negotiation
options can take (much) more room than the options on common DataAck packets.

Instead of reducing the MPS always for a case which only applies to the three
messages send during initial handshake, this patch devises a special case:

   if the payload length of the DataAck in PARTOPEN is too large, an Ack is sent
   to carry the options, and the feature-negotiation list is then flushed.

   This means that the server gets two Acks for one Response. If both Acks get
   lost, it is probably better to restart the connection anyway and devising yet
   another special-case does not seem worth the extra complexity.

The patch (over-)estimates the expected overhead to be 32*4 bytes -- commonly
seen values were 20-90 bytes for initial feature-negotiation options. 

It uses sizeof(u32) to mean "aligned units of 4 bytes". For consistency,
another use of sizeof is modified.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>