Added SCTP handling for masquerading

Added SCTP handling for masquerading
This was easier than expected... since masquerading changes an address,
but not the SCTP common header and further SCTP details, it does not
[seem to] pass into the SCTP checksum, as that uses no pseudo IP header AFAIK.
So, the [complicating] CRC-32 checksum computation need not be redone :-)
There is a need to update UDP signatures, also for an SCTP tunnel, and when
SCTP is the IPv6 payload there should be no checksum updated anywhere.
[We currently have too much code as a result of that, for sctp_offset.]