2.5.3:

* Options:
  - Allow parsing of boolean arguments (arguments with no value).
  - Really use the short names.  
  (Daniel Holbach, Murray Cumming)
* Added filename_display_basename() and filename_display_name().
  (Murray Cumming)
* Added setenv() and unsetenv(). (Murray Cumming)
* Use static keyword to prevent exporting private API.
  (Benoît Dejean)
* input example: Improvements and simplification. (Chris Vine)