Fix type 2.2 packet header generation

As reported by Deon on DOVE-Net: type 2.2 packet generation was not (since
SBBSecho v3.0) setting the origin and destination point values correctly:
was leaving the packet type 2.0 date values in these fields.

Also, zero-out the reserved bytes in the type 2.2 packet header (don't leave
the type 2.0 date values in there).

Thanks for the report Deon!