Thoughts on read/write support for exif header filesystem in jhead:

* Need to re-compact the files
* Need some method of adding directories and files
* Have some api that says "add this file to this directory", which creates 
  the directory if it doesn't already exist.
* There are different ways that the header can refer to subdirectories.  Need
  to handle all known ones, including GPS, makernotes, and interop dirs.


Make a nice C data structure for the directories.
At start, parse it onto the C data structure.
All data will be referred to by pointer, just point back into exif.
Have a sort of library for manipulating this directory structure.
Need some sort of double indirection for referring to other directories, as these
may have to be moved.  Maybe just a moderately sized table of pointers.
If directory structure is unchanged, can just write exif data straight back.
If directory structure changed, must recreate directories.