Use two sets of twilight points to represent segments.

Use two sets of twilight points to represent segments.

This is quite a large patch since almost everything depends on everything.

All points in the twilight zone have (0,0) as its origin, which means that
instructions like `MD_orig' produce meaningless results.  Consequently, a
second set of twilight points is introduced to hold the original point
coordinates.

Some other changes, besides fixing of thinkos:

o s/bci_loop_sal_assign/bci_set_up_segments/.  This also calls the code to
  create the twilight points.

o `bci_align_segments' now aligns to rp0 instead of a storage area value.
  Similarly, the already implemented actions use twilight points too.