bugfix: generate 32-bit site index in packets in a sane way

Use a sequential nonzero integer as site index instead of the address
of the site structure(!).  This value is not secret but does need to
be unique, and truncating the address to 32 bits doesn't guarantee
that.

Signed-off-by: Richard Kettlewell <richard@greenend.org.uk>
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>