uTP support for bitmask > 32 bits in selective ack

[kdeme]

From specification:

Selective ACK is an extension that can selectively ACK packets non-sequentially. Its payload is a bitmask of at least 32 bits, in multiples of 32 bits.

We currently only support 1x32bits, as the reference implementation does so too.

Not sure how much sense it makes to go further than this, but I've seen few times on the Portal network errors like this:

msg="Bad format of selective ack extension: extension = 0 len = 8"

Which indicate that other clients do allow more, and that it also occurs. Not sure if this is a healthy situation however, but that is another question.