Mark structures with UNALIGNED.

Maybe this will fix the crashes that appear to be occurring on the
opencsw.org buildbot; it's building with Sun/Oracle C, not GCC, but it's
at least worth a try.
Add {40,48,56}-bit big-endian extract macros and use them for DCCP ACKs.

Add macros to, given an octet pointer, extract 40-bit, 48-bit, and
56-bit big-endian numbers from the location pointed to by that pointer,
and use them when extracting ACK numbers from DCCP packets.  This fixes
problems on big-endian(!) machines.