Mark various IPv4 and IPv6 headers as unaligned.

Mark various IPv4 and IPv6 headers as unaligned.

This prevents GCC on SPARC from generating code that assumes those
structures are aligned naturally, which they are not guaranteed to be.

Move some #defines from interface.h to tcpdump-stdinc.h to make them
available to code that doesn't include interface.h.

Move the declaration of nextproto6_cksum() to ip6.h, so that only files
that use it get it declared, and thus so that you don't need to define
"struct ip6_hdr" in everything that includes interface.h.  Don't include
ip6.h in tcpdump-stdinc.h.