Introduce 3rd system/local date display format: YY/MM/DD

... and YYYY/MM/DD for birthdate input/display.

Now set in SCFG->System (not toggle options) and stored in the "date_fmt" key
(new) in main.ini. The old sys_misc SM_EURODATE flag is deprecated (but used
to determine the default value of date_fmt when not present).

As requested from: Max (WESTLINE)

  Is it possible to do a new dateformat in scfg
  In sweden we using YYYY-MM-DD format as standard.