Fix for 687625 Incorrect spot color behavior in tiffsep.

Fix for 687625 Incorrect spot color behavior in tiffsep.  Garbage
text was being printed for the spot color names.

DETAILS:

The problem was with the allocation and GC enumeration logic for the
storage for the storage for the spot color names.  Thus the problem
was only apparent after the GC relocation logic had executed.

The fix involves adding enumeration logic for the gs_param_string
structure and correcting the allocation of storage for the data.