00/01/28  EF
The mkblkdir option to use external structure definition
files uses the structure definitions only to convert
structures; they are not added to the structure definition
list in the new block files.  This is not easy to change.
It would require something like this:

   break convert_struct_def into two parts, one to convert
   from the old format to the host format, the second to go
   from the host format to the new format.  (The reason for
   the 2-part conversion is that strucdefs in the host
   format are needed for the data conversion process.)
   Between these two parts, insert a function to append the
   external structure definitions (which are in
   db->struct_def) to those from db->block_strdef, in such a
   way that storage is contiguous.  Count the new size of
   the structure definition block and update the new block
   header, noting that at this point the block header has
   not yet been converted, and the old block header info
   must not be changed.

In this case the addition would occur only when there is a
format conversion; to make it so that the additions would
always occur, a flag could probably be used to force the
conversion routines to be run.